Use the default dispatcher where possible for nested message loops.

Notable changes:
 * Add QuitNestedMessageLoop() to client::DispatcherClient, which can be used to
   terminate a nested loop started by RunWithDispatcher().
 * FirstRunDialog and SimpleMessageBoxViews are no longer
   MessagePumpDispatchers. The default dispatcher is used instead, and
   QuitNestedMessageLoop() is called to terminate the loop instead of returning
   POST_DISPATCH_QUIT_LOOP from the Dispatch() override.

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/182143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253723