Media Galleries API Metadata: Rename MediaFileSystemMetadata callback.

Just to disambiguate metadata callback for file systems vs. media files.

BUG=318450

Review URL: https://codereview.chromium.org/102723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238523 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238523