DevTools: expose layer compositing reasons and paint count via layer tree agent

This part only includes back-end and model changes.

- keep compositing reasons in GraphicsLayer;
- fix paint count calculation in GraphicsLayer (was always 0);
- drop {setShow,isShowing}RepaintCounter, they're unused;
- expose GraphicsLayer::replicatedLayer(), pretend it's a parent for replica layers.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23050018

git-svn-id: svn://svn.chromium.org/blink/trunk@156701 bbb929c8-8fbe-4397-9dbb-9b2b20218538