Media Galleries: Run API tests in a temp directory with an injected common.js file. Avoid the need for nested tests to share common JS code. (try 2)

Review URL: https://codereview.chromium.org/26798002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228034 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228034