CompositingReasonFinder shouldn't set dirty bits in RenderLayerCompositor

Now that we've got more of our compositing update house in order, we don't need
the gross hack wherein the CompositingReasonFinder sets a dirty bit in the
RenderLayerCompositor. In both cases where we would set this dirty bit, we now
correctly realize that we need to recompute the compositing requirements via
RenderLayerCompositor::m_updateType.

Review URL: https://codereview.chromium.org/312673004

git-svn-id: svn://svn.chromium.org/blink/trunk@175376 bbb929c8-8fbe-4397-9dbb-9b2b20218538