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/client_app_browser.h

@@ -66,6 +66,7 @@ class ClientAppBrowser : public ClientApp,
   CefRefPtr<CefPrintHandler> GetPrintHandler() OVERRIDE {
     return print_handler_;
   }
+  void OnScheduleMessagePumpWork(int64 delay) OVERRIDE;
 
   // Set of supported Delegates.
   DelegateSet delegates_;