DevTools: show owner node selectors in Layers tree

This replaces numeric layer ids with the selectors of their owning nodes.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22888003

git-svn-id: svn://svn.chromium.org/blink/trunk@156368 bbb929c8-8fbe-4397-9dbb-9b2b20218538