Currently the renderer is only ready when the input stream has more data to
read. Actual renderers, however, are also ready as long as their output buffer
contains some audio/video samples to play, roughly corresponding to the fact
that the playback time hasn't reached the timestamp of the last buffered
sample. Added a isready flag to FakeRenderer to simulate this behaviour.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162343074
