MSE: Allow requesting coded frame processor: legacy or new

MSE: Allow requesting coded frame processor: legacy or new

Updates WebMediaSource to have an addSourceBuffer() version that
allows caller to request which of LegacyFrameProcessor or a (not yet
landed in Chromium) new frame processor to use to process frames parsed
during appends to the source buffer.

The choice of which frame processor to request is based on
--enable-experimental-web-platform-features, as exposed through
RuntimeEnabledFeatures::mediaSourceExperimentalEnabled(): if enabled,
the new frame processor is requested. Otherwise, LegacyFrameProcessor is
requested.

Until the updated Chromium side WebMediaSourceImpl lands and (later)
the new Chromium frame processor lands, LegacyFrameProcessor will be
used regardless of the requested processor.

R=acolwell@chromium.org,abarth@chromium.org
BUG=249422
TEST=All MediaSource layout tests pass both before and after Chromium
side WMSI change is applied locally, and MediaSourceBase logging shows
frameProcessorChoice reflects usage (or lack) of cmdline switch
--enable-experimental-web-platform-features

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170678

[wolenetz, 2014-04-02 01:38:40]

PTAL @ Patch set 1:
acolwell@: everything
abarth@: public/platform/*

This is the first of a 3-sided change to introduce FrameProcessorChoice argument
to WebMediaSource::addSourceBuffer().

The second part is Chromium: https://codereview.chromium.org/220113002.

And the third part is Blink: https://codereview.chromium.org/220123011

[abarth-chromium, 2014-04-02 05:47:18]

public/ LGTM

[acolwell GONE FROM CHROMIUM, 2014-04-02 17:15:32]

lgtm

[wolenetz, 2014-04-02 17:23:07]

The CQ bit was checked by wolenetz@chromium.org

[commit-bot: I haz the power, 2014-04-02 17:23:18]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wolenetz@chromium.org/220593010/1

[commit-bot: I haz the power, 2014-04-02 18:32:18]

Change committed as 170678