Fix BraveShieldsAPIBrowserTest.AllowScriptsOnceIframe flakiness

[goodov]

BraveShieldsAPIBrowserTest.AllowScriptsOnceIframe test is flaky, fails for a long time but is always(?) successful on retries. This change possibly made the flakiness more obvious, but it's also reproduced easily without this change.

The test fails on the initial load step when all scripts should be blocked. It might be a test issue when a "block all" rule is not propagated to all renderers right away or something else.
Possibly related to upstream NavigationThreadingOptimizations feature.

[goodov]

The issue is that a spare renderer doesn't get ContentSettings updates after creation. Without NavigationThreadingOptimizations feature we were able to update rules here, but with the feature enabled, our code cannot do the rules update perfectly.

This looks like an edge case that Chromium doesn't handle, I'll try to upstream a fix for that.

[goodov]

Upstreaming it here https://chromium-review.googlesource.com/c/chromium/src/+/3439874

[iefremov]

We need a separate issue to rollback this change once upstream is fixed

[kjozwiak]

This was originally uplifted/scheduled to go into a 1.35.x HF but never ended up being released. It will go out via 1.36.x as we won't be having another 1.35.x release at this point.