Set the scrollbarLayer as opaque after setupScrollbarLayer.

setupScrollbarLayer passes scrollbarLayer to
GraphicsLayer::m_contentsLayer.
so if we want to change the property of scrollbarLayer,
we should set the property after calling setupScrollbarLayer.
currently we didn't set the scrollbarLayer as opaque.
this causes to draw unnecessary quads which are not
occluded in occlusion tracker.

Bug=None

Review URL: https://codereview.chromium.org/649563002

git-svn-id: svn://svn.chromium.org/blink/trunk@183946 bbb929c8-8fbe-4397-9dbb-9b2b20218538