Wrap window.ethereum._metamask in gin::Wrappable

[darkdh]

so that _metamask and ethereum have equivalent lifetime.

Resolves brave/brave-browser#30204

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
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  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, 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-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Described in the issue

[darkdh]

with owning mojo::Remote, we don't need weak_ptr because the previous bound callback won't be called when mojo disconnected

[kdenhartog]

Is it possible to avoid using base::Unretained(this) here? It seems like this opens up the possibility of another issue occurring if this object is destroyed before the async operation is completed based on what's being described here.

[darkdh]

This is intentional, when dealing with mojo interface, using weak_ptr is unnecessary. Mojo pipe will invalidate all of the ongoing message when disconnected so OnIsUnlocked won't be called when the class who owns mojo::Remote<mojom::EthereumProvider> is gone.

[kdenhartog]

LGTM

[kjozwiak]

Got the needed ++, going to merge so we can uplift into 1.51.x and get RC builds running for tomorrow.