Fixing broken graphics layer culling with 2D canvas

Currently, in cases where a canvas element's background cannot
be direct-composited, opaque backgrounds fail to render. This
is because the compositor thinks the canvas layer is opaque when
it is actually its container that is opaque. The opacity logic
was updated to correctly determine whether a canvas layer is
opaque.

BUG=412718
TEST=layout tests under compositing/draws-content + fast/canvas/bug412718.html

Review URL: https://codereview.chromium.org/569843002

git-svn-id: svn://svn.chromium.org/blink/trunk@182113 bbb929c8-8fbe-4397-9dbb-9b2b20218538