DevTools: report layers in layerTreeDidChange event, drop LayerAgent.getLayers()

Once layer tree gets updated lazily, building it at arbitrary moments of time
per front-end request may become complicated. So let's push it in layerTreeDidChange
instead. Also, do not re-enable LayerTreeAgent automatically upon navigation, let
front-end request this explicitly after it gets document from the DOMAgent.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/26897003

git-svn-id: svn://svn.chromium.org/blink/trunk@160813 bbb929c8-8fbe-4397-9dbb-9b2b20218538