DevTools: untangle layer views from LayerTreeModel for better reuse

- extract building of layer trees into TracingLayerTree and AgentLayerTree;
- LayerTreeModel now is only a light InspectorLayerTreeAgent client, with
    static snapshot logic removed into *LayerTree;
- make all layer views use LayerTreeBase, not LayerTreeModel.

BUG=

Review URL: https://codereview.chromium.org/294043002

git-svn-id: svn://svn.chromium.org/blink/trunk@174480 bbb929c8-8fbe-4397-9dbb-9b2b20218538