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This CL refactors the PlayerCompositorDelegate to structure it in a way
that is more amenable to testing. This CL also adds a significant amount
of test coverage for PlayerCompositorDelegate.

PlayerCompositorDelegateAndroid is refactored to be compatible with the
changes to PlayerCompositorDelegate, but it is not tested as it is
relatively well tested from the Java side.

This boosts coverage to > 85%. Further coverage is difficult as the
edges are a result of difficult to simulate failures.

Bug: 1128927
Change-Id: Id7467209ab2858043655ae374e55ac699e4e1778
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412893Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807555}