Move iframe and plugin compositing triggers into additionalCompositingReasons

This CL move the computation of iframe and plugin compositing triggers into
iframe- and plugin-specific overrides of
RenderObject::additionalCompositingReasons. This approach means that we don't
need to take subtype specific branches for every type in
CompositingReasonFinder.

Review URL: https://codereview.chromium.org/199443009

git-svn-id: svn://svn.chromium.org/blink/trunk@169701 bbb929c8-8fbe-4397-9dbb-9b2b20218538