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Convert the media and audio UI test to a browser_test. browser_tests are sharded and run quicker, and are generally less flaky than ui tests. They'll also be portable to content_browsertest once we have it.

BUG=90448,117828
Review URL: https://chromiumcodereview.appspot.com/9959028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129958 0039d316-1c4b-4281-b951-d872f2087c98