DevTools: pick style rule selector from DOM outline

The patch implements capability to pick a node from DOM outline
during style rule selector editing so that the selector will be generated according
to selected node.

BUG=406096

Review URL: https://codereview.chromium.org/490273002

git-svn-id: svn://svn.chromium.org/blink/trunk@180795 bbb929c8-8fbe-4397-9dbb-9b2b20218538