A synthetic mask layer is not needed if there is no content under
it which draws. This patch implements that optimization, and also
adds a new optimization to CompositedLayerMapping to set DrawsContent
to false for layers which have empty size.

The patch also removes one unused method from cc::Layer.

Bug: 947715

Change-Id: I7625fc251a3dc04ce5319edc56a9dc5fccf27564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566029
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650514}