Port ChannelDestructionWatcher in PrerenderBrowserTests to base::RunLoop.

Saves us from reimplementing various cases about Quit being called
before Run.

BUG=307592

Review URL: https://codereview.chromium.org/26299007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228845 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228845