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_cpptoc_impl.py

@@ -613,9 +613,15 @@ def make_cpptoc_class_impl(header, clsname, impl):
   # any derived classes can be unwrapped
   unwrapderived = make_cpptoc_unwrap_derived(header, cls, base_scoped)
 
+  const =  '// CONSTRUCTOR - Do not edit by hand.\n\n'+ \
+           clsname+'CppToC::'+clsname+'CppToC() {\n'
+  const += make_cpptoc_virtual_function_assignment(header, cls, prefixname,
+                                                   defined_names)
+  const += '}\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))
 
@@ -626,12 +632,6 @@ def make_cpptoc_class_impl(header, clsname, impl):
 
   parent_sig = template_class + '<' + clsname + 'CppToC, ' + clsname + ', ' + capiname + '>'
 
-  const =  '// CONSTRUCTOR - Do not edit by hand.\n\n'+ \
-           clsname+'CppToC::'+clsname+'CppToC() {\n'
-  const += make_cpptoc_virtual_function_assignment(header, cls, prefixname,
-                                                   defined_names)
-  const += '}\n\n'
-
   if base_scoped:
     const += 'template<> CefOwnPtr<'+clsname+'> '+parent_sig+'::UnwrapDerivedOwn(CefWrapperType type, '+capiname+'* s) {\n' + \
              unwrapderived[0] + \