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.

include/internal/cef_types.h

@@ -180,6 +180,18 @@ typedef struct _cef_settings_t {
   ///
   int multi_threaded_message_loop;
 
+  ///
+  // Set to true (1) to control browser process main (UI) thread message pump
+  // scheduling via the CefBrowserProcessHandler::OnScheduleMessagePumpWork()
+  // callback. This option is recommended for use in combination with the
+  // CefDoMessageLoopWork() function in cases where the CEF message loop must be
+  // integrated into an existing application message loop (see additional
+  // comments and warnings on CefDoMessageLoopWork). Enabling this option is not
+  // recommended for most users; leave this option disabled and use either the
+  // CefRunMessageLoop() function or multi_threaded_message_loop if possible.
+  ///
+  int external_message_pump;
+
   ///
   // Set to true (1) to enable windowless (off-screen) rendering support. Do not
   // enable this value if the application does not use windowless rendering as

include/internal/cef_types_wrappers.h

@@ -575,6 +575,7 @@ struct CefSettingsTraits {
         src->browser_subprocess_path.length,
         &target->browser_subprocess_path, copy);
     target->multi_threaded_message_loop = src->multi_threaded_message_loop;
+    target->external_message_pump = src->external_message_pump;
     target->windowless_rendering_enabled = src->windowless_rendering_enabled;
     target->command_line_args_disabled = src->command_line_args_disabled;