CompositingReasonFinder should use a virtual function instead of a nest of branches

This CL begins to move CompositingReasonFinder to using a virtual function on
RenderObject instead of a nest of "if video, if canvas" branches. This CL only creates
two overrides, one for video and one for canvas. A future CL will create
overrides for plugins and iframes, but there's some cleanup work we need to do first.

Review URL: https://codereview.chromium.org/197533013

git-svn-id: svn://svn.chromium.org/blink/trunk@169685 bbb929c8-8fbe-4397-9dbb-9b2b20218538