MediaRecorder: support sampling rate adaption in AudioTrackRecorder

This CL adds support for audio coming from the MediaStream
Tracks with a sampling rate not directly supported by Opus
({48k, 24k, 16k, 12k, 8k}samples/s) by adding a AudioConverter
in AudioTrackRecorder::AudioEncoder.

- unit tests, which are beefed up
- content_browsertests
- demo [1]
- corrected bug's file [2]
(correction being: s/opus/webm/ for the mimeType to
use for capture.)
  I used a variety of input wav's, mostly the original
 sfx5.wav in the bug and the "miscellaneous" entries in [3]


Review URL:

Cr-Commit-Position: refs/heads/master@{#372491}
4 files changed