Avoid passing uninitialized value to markRectAsNonOpaque.

While applying opaque region for layer if device clip is not a rect 
we don't alter the opaque rect, and if we have a non-opaque
preserving transfer mode along with it, we mark the opaque rect
as empty.

BUG=390999

Review URL: https://codereview.chromium.org/417153002

git-svn-id: svn://svn.chromium.org/blink/trunk@179741 bbb929c8-8fbe-4397-9dbb-9b2b20218538