DevTools: Add workerShadowPageLoaded method to shared/service worker clients for further plumbing into the browser.

This method is going to be used as a new signal for opening worker frontend when 'debug on start' option is enabled.

BUG=404355

Review URL: https://codereview.chromium.org/483813002

git-svn-id: svn://svn.chromium.org/blink/trunk@180480 bbb929c8-8fbe-4397-9dbb-9b2b20218538