Add some incremental compositing tests

These tests that compositing updates work even when the render tree changes in
a minor way. These tests would have failed when introducing dirty bits for
CompositedLayerMapping in https://codereview.chromium.org/183763016.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/183833016

git-svn-id: svn://svn.chromium.org/blink/trunk@168358 bbb929c8-8fbe-4397-9dbb-9b2b20218538