DevTools: Selection/hover support in both tree and 3d views of Layers panel

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22836003

git-svn-id: svn://svn.chromium.org/blink/trunk@156366 bbb929c8-8fbe-4397-9dbb-9b2b20218538