Reland 129925 - ash: Icon cache for app list to save shadow generation time.

Reland 129925 - ash: Icon cache for app list to save shadow generation time.

- Add an IconCache that stores processed (resized and shadow-generated) image,
  keyed by source image + desired size;
- The cache is created with AppList controller and destroyed when controller is
  gone;
- Controller marks all entries in cache unused before showing the UI and purges
  all unused entry after dismissing UI;
- In item view, try cache first to get processed image before scheduling job,
  and update cache when background processing comes back;

BUG=120961
TEST=Verify fix for issue 120961.

Review URL: http://codereview.chromium.org/9934006

TBR=sky@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9968024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@130035 0039d316-1c4b-4281-b951-d872f2087c98