DevTools: add Reveal in Elements panel option to layer context menu

- handle DOMNode targets in appendApplicableItems() of Element panel and its descriptor;
- handle contextmenu event in LayerTree and Layers3DView;
- dispatch appendApplicableItems with a DOMNode of layer owner from the tree and the 3d view;

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23137017

git-svn-id: svn://svn.chromium.org/blink/trunk@156845 bbb929c8-8fbe-4397-9dbb-9b2b20218538