- Don't send a timeline to the listener until all children
have reported their timelines.
- Propagate a proper merge error if merging fails.
- The PlayerActivity hack is necessary due to the way Andorid's
MediaController widget attaches to the window :(. It'll go
away once we get our own player controls.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131958169
