Remove cannotBlitToWindow

The concept of blitting to the window doesn't make sense now that the world is
composited.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/441113005

git-svn-id: svn://svn.chromium.org/blink/trunk@179663 bbb929c8-8fbe-4397-9dbb-9b2b20218538