Cleanup: Move MediaGalleriesCustomBindings::GetMediaFileSystemObject() into an anonymous namespace.

Review URL: https://codereview.chromium.org/129573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243723