Add WebCompositorSupport bindings for nine-patch layers.

These will be used to composite root content layer shadows when
over-scrolling on Mac.

BUG=133097

Review URL: https://chromiumcodereview.appspot.com/22864004

git-svn-id: svn://svn.chromium.org/blink/trunk@156047 bbb929c8-8fbe-4397-9dbb-9b2b20218538