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/base/cef_macros.h

@@ -39,6 +39,9 @@
 // Chromium uses movable types.
 #define MOVE_SCOPED_PTR(var) std::move(var)
 
+// Chromium uses std types.
+#define DEFAULT_DELETER(type) std::default_delete<type>
+
 #else  // !USING_CHROMIUM_INCLUDES
 // The following is substantially similar to the Chromium implementation.
 // If the Chromium implementation diverges the below implementation should be
@@ -50,6 +53,9 @@
 // CEF does not use movable types.
 #define MOVE_SCOPED_PTR(var) var.Pass()
 
+// CEF uses base types.
+#define DEFAULT_DELETER(type) struct base::DefaultDeleter<type>
+
 #if !defined(arraysize)
 
 // The arraysize(arr) macro returns the # of elements in an array arr.