Add browset tests for webaudio using media stream.

This is the browser test for https://codereview.chromium.org/85523002, which is about Fixing the crash in the webaudio source provider when the audio track is going away.

BUG=322554
TEST=content_browsertests

Review URL: https://codereview.chromium.org/101783003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238704 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238704