The position of opaque black rect is incorrect when alpha is false.

It should be drawn using the layout rect instead of the canvas size rect.

BUG=382884

Review URL: https://codereview.chromium.org/327703005

git-svn-id: svn://svn.chromium.org/blink/trunk@176024 bbb929c8-8fbe-4397-9dbb-9b2b20218538