Notify WebM media segment start when kWebMIdCluster is parsed
Fixes WebM stream parser to conform more closely to MSE segment loop
parser's PARSING_MEDIA_SEGMENT state entrance:
Immediately calls the new segment CB when |kWebMIdCluster| is parsed,
instead of waiting until enough data has been appended and parsed
for the cluster parser to determine the cluster start timestamp.
Adds associated unit tests (for WebM).
R=acolwell@chromium.org
BUG=334127
TEST=All media_unittests and MSE layout tests pass locally on Linux
Review URL: https://codereview.chromium.org/
133433005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245728 0039d316-1c4b-4281-b951-d872f2087c98