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).

BUG=569976, 575492

TEST= run chromium with
 against e.g.

Cr-Commit-Position: refs/heads/master@{#395205}
15 files changed