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

This change is the first in a three-sided set of changes, and introduces
WebSourceBufferClient for handling media engine notifications
of newly received initialization segments.
It adds logic to make the addition of a SourceBuffer into
MediaSource.activeSourceBuffers more compliant once all three sides
land: addition will be done per the MSE spec's first initialization
flag logic rather than other non-compliant approaches like at
addSourceBuffer() time or on transition to HAVE_METADATA. Insertion
logic is included to preserve correct ordering of
activeSourceBuffers.
Updated and new layout tests are included.

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/552943002

git-svn-id: svn://svn.chromium.org/blink/trunk@181861 bbb929c8-8fbe-4397-9dbb-9b2b20218538