DevTools: Revamp Layer agent, reduce it to what we'll actually use.

This trims layer agent down to what we will actually be using in the near term.
Also:
- return properties of GraphicsLayer, not RenderLayer;
- layer ids are now that of WebLayer, so we can eventually match to those
    in Timeline;
- compositing reasons are gone, but will eventually be back;
- explicit support for generated nodes is gone, should be generic via DOMAgent.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22577005

git-svn-id: svn://svn.chromium.org/blink/trunk@155776 bbb929c8-8fbe-4397-9dbb-9b2b20218538