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There is one known precondition failure that is added to WPTOverrideExpectations.
Others will be added in a follow-up Cl.

Also expands unit tests to check that we're parsing trailing comments
into annotations properly.

Bug: 937369
Change-Id: I325272eee364b8cae9ef7739ff9333b829f9f041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088234
Commit-Queue: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747269}