Create chrome://serviceworker-internals

This is the beginning of the internals page for service worker, to aid in
debugging and development. This lists all registered service workers and
allows you to manually start/stop the workers, as well as unregister them.

BUG=351197

Review URL: https://codereview.chromium.org/182383008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256858 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256858