The right way to detect the presence of a keyframe in a BlockGroup
is to look for the absence of the ReferenceBlock element [1].
Closes b/30433206
[1] https://matroska.org/technical/specs/index.html#ReferenceBlock
Change-Id: I2b3c8704cbabcd5cbee38448e5528425865561a4
Also includes its dependencies, like media/base/text_track_config.*
and media/filters/webvtt_util.h
Change-Id: I5b26245daf004da19b912b7c5b2c21ce4ba85688