(Fixing GitHub issue #2923)
Cuurently, ClippingMediaSource issues an initial discontinuity. This causes the
renderers to be disabled and re-enabled when this media source is used in a sequence
with other sources (or in a loop).
This change disables the use of an initial discontinuity for audio-only media under
the assumption that audio streams have random access capabilities.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159221963
