MSE: Complete letting SourceBuffer begin to do initialization segment received algorithm

This change is the third in a three-sided set of changes, and lets
WebSourceBufferClient be the sole trigger for Blink adding a
SourceBuffer into SourceBuffer.activeSourceBuffers.

R=acolwell@chromium.org,philipj@opera.com,tkent@chromium.org
TEST=No mediasource layout test regression
BUG=249428, 397243

Review URL: https://codereview.chromium.org/562503002

git-svn-id: svn://svn.chromium.org/blink/trunk@181949 bbb929c8-8fbe-4397-9dbb-9b2b20218538