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Fixes a test flake that was caused by incomplete precondition
criteria. Now the test case will wait until the document is parsed
before executing the test case.

Bug: 1162433
Test: Executed test case with --gtest_repeat=500, no issues found.
Change-Id: I0c5f58965906ec4e83b6212287a529d5b0190034
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638333
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845274}