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.

libcef_dll/ctocpp/display_handler_ctocpp.h

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=309e3816e3daba60c7809230fdecb59d01e41847$
+// $hash=6d8ab7f2086373de52eb178cc640d2d553d43c58$
 //
 
 #ifndef CEF_LIBCEF_DLL_CTOCPP_DISPLAY_HANDLER_CTOCPP_H_
@@ -51,6 +51,8 @@ class CefDisplayHandlerCToCpp
                         const CefString& message,
                         const CefString& source,
                         int line) override;
+  bool OnAutoResize(CefRefPtr<CefBrowser> browser,
+                    const CefSize& new_size) override;
 };
 
 #endif  // CEF_LIBCEF_DLL_CTOCPP_DISPLAY_HANDLER_CTOCPP_H_