* Use correct bounds to pick and resize wallpaper image
* Use separate layer to draw wallpaper and scale it,
rather than scaling at drawing time, which uses more CPU and memory.
* Changed ctrl-alt-b behavior so that it uses fixed image with different layouts.

BUG=294479
TEST=manual + unittests.
R=derat@chromium.org

Review URL: https://codereview.chromium.org/24269015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225116 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225116