Add rewards page initialization check

[zenparsing]

Resolves brave/brave-browser#10717

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Test case in brave-browser issue.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[emerick]

LGTM

[LaurenWags]

Verified using

Brave | 1.14.18 Chromium: 84.0.4147.105 (Official Build) nightly (64-bit)
-- | --
Revision | a6b12dfad6663f13a7e16e9a42a6a4975374096b-refs/branch-heads/4147@{#943}
OS | macOS Version 10.14.6 (Build 18G3020)

Upgrades from 1.10.x --> 1.14.x

• Verified scenario 1 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page closed prior to upgrade. Confirmed all other modified AC settings are maintained.
• Verified scenario 1 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page open/in focus prior to upgrade. Note, while wallet migration is occurring the Monthly Payment is blank for a few seconds but then it populates. Confirmed all other modified AC settings are maintained.
• Verified scenario 2 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page closed prior to upgrade.
• Verified scenario 2 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page open/in focus prior to upgrade. Note, while wallet migration is occurring the Monthly Payment is blank for a few seconds but then it populates.

Note - Also observed panel shows Join messaging when upgrading from 1.10.x to 1.14.x. This also occurs with 1.10.x to 1.12.x and is not related to this PR. Investigation was done with @NejcZdovc and was shared here: https://bravesoftware.slack.com/archives/CB0KCRCQN/p1596633919026400?thread_ts=1596630882.025600&cid=CB0KCRCQN

Also checked the below using 1.11.x --> 1.14.x

• Verified scenario 1 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page closed prior to upgrade. Confirmed all other modified AC settings are maintained.
• Verified scenario 1 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page open/in focus prior to upgrade. Confirmed all other modified AC settings are maintained.
• Verified scenario 2 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page closed prior to upgrade.
• Verified scenario 2 from [Desktop] monthly payment isn't always displayed on brave://rewards correctly - follow up to 10438 brave-browser#10717 (comment) with brave://rewards page open/in focus prior to upgrade.