CompositedLayerMapping should use dirty bits for bounds update

This CL teaches CompositedLayerMapping::updateCompositedBounds about the
m_needToUpdateGeometry dirty bit. Now we only update the composited bounds if
they could have possibly changed.

This CL saves 14% of the compositing update in Polymer's calculator sample app.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/208643002

git-svn-id: svn://svn.chromium.org/blink/trunk@169762 bbb929c8-8fbe-4397-9dbb-9b2b20218538