DevTools: descend into iframes when collecting layers

We used to walk RenderLayer hierarchy while collecting layers and
were only visitng those GraphicsLayers referred from RenderLayerBacking.
Now we build node to layer id map by walking RenderLayer hierarchy,
then collect the GraphicsLayer tree and assign node ids based on the
map. Note we still skip top-most layers not related to any node on the
presentation level.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23619007

git-svn-id: svn://svn.chromium.org/blink/trunk@157214 bbb929c8-8fbe-4397-9dbb-9b2b20218538