[MediaGalleries] Move MTP tracking from the ExtensionGalleriesHost to the registry.

R=vandebo@chromium.org

Review URL: https://codereview.chromium.org/26881002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228573