Fix shell_unittests flakes related to external_view_embedder

[cyanglaz]

Description

The tests were still flaky, see: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8863001188407459360/+/steps/Host_Tests_for_host_profile/0/stdout?format=raw

Although I couldn't repro this locally or on a cloud linux box, there is clearly a race condition that might cause the same failure, due to the below reason.
There was an automatically resubmitted frame due to kResubmitFrame. And the failed check is not running on raster thread, so it could happen before or after the 2nd frame's SubmitFrame is called. Hence the flake.

The solution is to remove those similar checks, which is really unnecessary. Instead, we just need to make sure there is only a single external_view_embedder->GetSubmittedFrameCount() during the 2 frames before and after resubmitting.

Related Issues

Fixes flutter/flutter#66056

Tests

I added the following tests:

Updated tests such as:
ShellTest::SkipAndSubmitFrame
ShellTest::ReSubmitFrame
ShellTest::DiscardLayerTreeOnResize

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Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

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[iskakaushik]

LGTM