DevTools: add a method for resuming target to WebDevToolsAgentClient and use it in RuntimeAgent.run.

We plan to (temporary) implement service worker debugging through pausing its setup process, connecting to it, set up inspector backend state and resuming then. 

R=pfeldman-OOO, yurys
BUG=404355

Review URL: https://codereview.chromium.org/490033004

git-svn-id: svn://svn.chromium.org/blink/trunk@181036 bbb929c8-8fbe-4397-9dbb-9b2b20218538