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

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.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169685

[ojan, 2014-03-20 17:52:55]

lgtm

[commit-bot: I haz the power, 2014-03-20 17:52:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/abarth@chromium.org/197533013/1

[commit-bot: I haz the power, 2014-03-20 18:01:38]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-20 18:01:38]

Try jobs failed on following builders:
  tryserver.blink on mac_blink_compile_dbg

[abarth-chromium, 2014-03-20 18:38:40]

The CQ bit was checked by abarth@chromium.org

[commit-bot: I haz the power, 2014-03-20 18:38:50]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/abarth@chromium.org/197533013/20001

[commit-bot: I haz the power, 2014-03-20 19:56:47]

Change committed as 169685