Start of a typing detection test: mixes webaudio with getUserMedia.

This test will request user media, load an audio file from webaudio and mix both together. It sends the combined stream into an audio-only WebRTC call which is running in two tabs.

This patch is just the first step. The test is currently disabled because it isn't complete. The next steps are to
1) inject fake key events
2) listen to the typing-is-detected state and succeed iff the state changes from no-typing-detected to typing-detected.

BUG=308138

Review URL: https://codereview.chromium.org/27112007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229872 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229872