Remove DISALLOW_ macro from libcef/ (see issue #3234)

Also perform related C++ cleanup:
- Use =default instead of {} for default implementations of
  constructors/destructors.
- Replace typedef with using.

libcef/renderer/render_manager.cc

@@ -28,6 +28,7 @@
 #include "libcef/renderer/v8_impl.h"
 
 #include "base/command_line.h"
+#include "base/macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "cef/libcef/common/mojom/cef.mojom.h"
 #include "content/public/common/content_switches.h"