(Relating to GitHub Issue #2577)
The Timeline base class provides the default implementation.
Timeline wrappers (e.g. ClippingTimeline, ConcatatedTimeline) forward all
requests to the respective inner timelines. Some like ConcatenatedTimeline add
their own additional logic to bridge between the child timelines.
In addition, ConcatenatedTimeline and LoopingTimeline now have a common
abstract base class as they share most of their code.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=155509269
