Remove unused methods of GraphicsLayerClient

didCommitChangesForLayer() & getCurrentTransform() were apparently needed
by other platforms and are unused in blink.

Review URL: https://codereview.chromium.org/116223004

git-svn-id: svn://svn.chromium.org/blink/trunk@164078 bbb929c8-8fbe-4397-9dbb-9b2b20218538