Fixed preloading thumbnails while copying.

The previous code was broken by using a wrong enum name, and by fetching the metadata in an incorrect way.

BUG=328238
TEST=Tested manually.

Review URL: https://codereview.chromium.org/113913007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241251 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241251