Improve support for a host owned message pump (issue #1805)

- Add new CefSettings.external_message_pump option and
  CefBrowserProcessHandler::OnScheduleMessagePumpWork() callback.
- Improve documentation related to CefDoMessageLoopWork().
- Pass `--external-message-pump` command-line flag to cefclient or
  cef_unittests to test the new mode.

tests/cefclient/browser/main_message_loop.h

@@ -51,7 +51,7 @@ class MainMessageLoop {
 
  protected:
   // Only allow deletion via scoped_ptr.
-  friend struct base::DefaultDeleter<MainMessageLoop>;
+  friend DEFAULT_DELETER(MainMessageLoop);
 
   MainMessageLoop();
   virtual ~MainMessageLoop();