cc: Wait for callback indicating that copy is complete
cc: Wait for callback indicating that copy is complete

This should fix the failures (the callback isn't in when the test
completes) and crashes (the callback lands after the test has torn down)
plaguing LayerTreeHostCopyRequestCompletionCausesCommit.

BUG=567439
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1512163002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364276}

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