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While debugging, we found that a ReportBeginImpl(Main)Frame could
happen before ScheduleTerminate, and then that main/impl frame is
never processed (meaning that it didn't report no damage and didn't
submit the frame).

This CL handles the impl case, main frame handling will be in a
separate CL.

Bug: 1021963
Change-Id: Ib9a1bd7b894bba3018f120590511c6dba22d1d00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994086Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730205}