Migrate from URLs to Entries in the Files App's gallery.

This patch migrates from URLs in favor of Entries, wherever possible.

TEST=Tested manually. Renamed, modified an image, deleted an image.
BUG=326901

Review URL: https://codereview.chromium.org/109973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240098 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240098