Brave News Display Ad gets served more than once

[petemill]

Test plan

See brave/brave-core#9743

Description

Occurs for first ad position - ad content will get requested twice. The first response will be discarded and the second response will be used. This results in premature ad capping from the ads service.

The only way to monitor the issue on the current build is to check the ads service local SQL database. The fix will contain more debug logging.

[stephendonner]

Verified PASSED using

Brave	1.29.67 Chromium: 92.0.4515.159 (Official Build) beta (x86_64)
Revision	0185b8a19c88c5dfd3e6c0da6686d799e9bc3b52-refs/branch-heads/4515@{#2052}
OS	macOS Version 11.5.2 (Build 20G95)

Steps:

1. new profile
2. launched Brave
3. opened the "hamburger" menu, and chose More Tools -> Developer Tools
4. from the All levels dropdown, added Verbose
5. opened a new-tab page
6. scrolled down
7. examined console

Confirmed I only saw one asking for an ad 0 call logged in the Console