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Previously, these tests would occasionally flake while waiting for the
scroll offset to update. This CL rewrites these tests to remove this
dependency on RenderFrameSubmissionObserver::WaitForScrollOffset.

This CL also changes these tests to use JavaScript promises to
communicate with the page instead of polling the console output in order
to improve readability of the tests.

Fixed: 1035057
Change-Id: I845d0f821bdc4ee604a9432a90f5e4a100187c97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166633
Commit-Queue: Scott Little <sclittle@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762789}