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.cc

@@ -6,6 +6,7 @@
 
 #include "include/base/cef_logging.h"
 #include "include/cef_cookie.h"
+#include "cefclient/browser/main_message_loop_external_pump.h"
 #include "cefclient/common/client_switches.h"
 
 namespace client {
@@ -82,4 +83,12 @@ void ClientAppBrowser::OnRenderProcessThreadCreated(
     (*it)->OnRenderProcessThreadCreated(this, extra_info);
 }
 
+void ClientAppBrowser::OnScheduleMessagePumpWork(int64 delay) {
+  // Only used when `--external-message-pump` is passed via the command-line.
+  MainMessageLoopExternalPump* message_pump =
+      MainMessageLoopExternalPump::Get();
+  if (message_pump)
+    message_pump->OnScheduleMessagePumpWork(delay);
+}
+
 }  // namespace client