MediaCaptureFromElement: add support for audio captureStream().
This CL extends support for capturing the audio part of
a <video> or <audio> tags ( "capture" here means creating
a MediaStream out of the HTMLElement)
It introduces an HtmlAudioCapturerSource is-a AudioCapturerSource
wrapped into an ExternalMediaStreamAudioSource to produce data
towards the audio track.
HtmlAudioCapturerSource also plugs into the
WebMediaPlayer's WebAudioSourceProviderImpl to get
a copy of the audio being rendered.
Unit tests are added, and the existing LayouTests
revamped (and split into several files for clarity).
TEST= run chromium with
15 files changed