CSS clip-path should prevent direct image compositing

Directly composited images with clip-path style do not render correctly
unless the clip-path is drawn in software (issue 408460).

If a render image has clip-path style, deny direct image compositing to
make the image layer draw its own content. Add a layout test.

  compositing/images/direct-image-clip-path.html

A test for dynamic addition of clip-path style to a directly composited
image will be added in issue 405803.

TBR=dstockwell@chromium.org
BUG=408460, 405803

Review URL: https://codereview.chromium.org/522673002

git-svn-id: svn://svn.chromium.org/blink/trunk@181138 bbb929c8-8fbe-4397-9dbb-9b2b20218538