Disable MediaGalleriesPlatformAppBrowserTest.MediaGalleriesCopyTo for being flaky.

BUG=314576

Review URL: https://codereview.chromium.org/46383004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232694 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232694