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As the second of a three-sided set of change:
This change lays the groundwork for Blink to own the app-visible pieces
of the MSE initialization segment received algorithm. It lets
SourceState notify WebSourceBufferImpl upon parsing an initialization
segment. WSBI then notifies its WebSourceBufferClient.
Updated unit tests are included.

R=acolwell@chromium.org
TEST=No media_unittest regression
BUG=249428,397243

Review URL: https://codereview.chromium.org/547223003

Cr-Commit-Position: refs/heads/master@{#294722}