Enable content_browsertest MediaSourceTest on Android.

These tests pass on my Nexus 7 v2. Occasionally I see a timeout in MediaSourceTest.ConfigChangeVideo test but the next retry passes. I'll check if the same thing happens on the test bots later.

BUG=304956
TEST=MediaSourceTest work on Nexus 7 v2.
R=qinmin@chromium.org

Review URL: https://codereview.chromium.org/60433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233188 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233188