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_context_impl.cc

@@ -113,6 +113,11 @@ void MainContextImpl::PopulateSettings(CefSettings* settings) {
       command_line_->HasSwitch(switches::kMultiThreadedMessageLoop);
 #endif
 
+  if (!settings->multi_threaded_message_loop) {
+    settings->external_message_pump =
+        command_line_->HasSwitch(switches::kExternalMessagePump);
+  }
+
   CefString(&settings->cache_path) =
       command_line_->GetSwitchValue(switches::kCachePath);