Create a separate layer for overhang shadows on Mac

Add a SetContentsToNinePatchLayer to GraphicsLayer.

Create a new layer in RenderLayerCompositor for the nine-patch layer.

Cut paintOverhangAreas into paintOverhangBackground and
paintOverhangShadows. The non-composited path still calls both
of these but the composited path just calls paintOverhangBackground
(for now -- it will do neither when the background is replaced with a
layer, or we use a pattern for gutters, etc).

BUG=133097

Review URL: https://chromiumcodereview.appspot.com/23050013

git-svn-id: svn://svn.chromium.org/blink/trunk@156677 bbb929c8-8fbe-4397-9dbb-9b2b20218538