GitLab

The CL moves the caches in Gallery items to:

 * Simplify UI class.
 * Share the cache different UI classes.
 * Move loading logic into the items in future patches.

BUG=391643
TEST=manually

Review URL: https://codereview.chromium.org/420743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285890 0039d316-1c4b-4281-b951-d872f2087c98