Use the default dispatcher where possible for nested message loops.

* SimpleMessageBoxViews is no longer MessagePumpDispatcher.
  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.

This is a partial re-land of crrev.com/253723

BUG=351393
TEST=trybot and manual testing on Chrome OS.
R=msw@chromium.org, sadrul@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/197903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256666 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256666