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

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181949

[wolenetz, 2014-09-10 00:44:38]

PTAL @ PS1:
acolwell@, philipj@: everything
abarth@: OWNERS review for public/

This is third of 3-sided blink->chromium->blink set of changes:
Part 1: https://codereview.chromium.org/552943002/
Part 2: https://codereview.chromium.org/547223003/
Part 3: https://codereview.chromium.org/562503002/

[philipj_slow, 2014-09-10 13:14:18]

lgtm

[wolenetz, 2014-09-10 21:19:57]

PS2 is just a rebase onto the updated (PS5) of part 1 of this 3-part change.
PTAL, same reviewers.

[philipj_slow, 2014-09-11 14:55:17]

Still LGTM. If a rebase doesn't introduce any interesting changes, it's OK to go
ahead without a second review. OK by me, at least :)

[wolenetz, 2014-09-11 22:57:12]

wolenetz@chromium.org changed reviewers:
+ jamesr@chromium.org
- abarth@chromium.org

[wolenetz, 2014-09-11 23:09:49]

wolenetz@chromium.org changed reviewers:
+ tkent@chromium.org
- jamesr@chromium.org

[wolenetz, 2014-09-11 23:12:41]

Both abarth@ and jamesr@ have indicated their unavailability to CR the
public/platform piece of this CL.
tkent@, please CR public/platform.
Thank you!

[tkent, 2014-09-12 00:28:18]

lgtm.

[wolenetz, 2014-09-13 02:04:03]

The CQ bit was checked by wolenetz@chromium.org

[commit-bot: I haz the power, 2014-09-13 02:04:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/562503002/60001

[commit-bot: I haz the power, 2014-09-13 03:34:08]

Committed patchset #4 (id:60001) as 181949