Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

tools/make_ctocpp_impl.py

@@ -610,9 +610,13 @@ def make_ctocpp_class_impl(header, clsname, impl):
   # any derived classes can be unwrapped
   unwrapderived = make_ctocpp_unwrap_derived(header, cls, base_scoped)
 
+  const =  '// CONSTRUCTOR - Do not edit by hand.\n\n'+ \
+           clsname+'CToCpp::'+clsname+'CToCpp() {\n'+ \
+           '}\n\n'
+
   # determine what includes are required by identifying what translation
   # classes are being used
-  includes = format_translation_includes(header, resultingimpl +
+  includes = format_translation_includes(header, const + resultingimpl +
                                          (unwrapderived[0]
                                           if base_scoped else unwrapderived))
 
@@ -623,10 +627,6 @@ def make_ctocpp_class_impl(header, clsname, impl):
 
   parent_sig = template_class + '<' + clsname + 'CToCpp, ' + clsname + ', ' + capiname + '>'
 
-  const =  '// CONSTRUCTOR - Do not edit by hand.\n\n'+ \
-           clsname+'CToCpp::'+clsname+'CToCpp() {\n'+ \
-           '}\n\n'
-
   if base_scoped:
     const += 'template<> '+capiname+'* '+parent_sig+'::UnwrapDerivedOwn(CefWrapperType type, CefOwnPtr<'+clsname+'> c) {\n'+ \
              unwrapderived[0] + \