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/cpptoc/extension_cpptoc.h

@@ -0,0 +1,40 @@
+// Copyright (c) 2017 The Chromium Embedded Framework Authors. All rights
+// reserved. Use of this source code is governed by a BSD-style license that
+// can be found in the LICENSE file.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool. If making changes by
+// hand only do so within the body of existing method and function
+// implementations. See the translator.README.txt file in the tools directory
+// for more information.
+//
+// $hash=a3c31e0a1f6ee88b5d5f908f95bc326a08baecf1$
+//
+
+#ifndef CEF_LIBCEF_DLL_CPPTOC_EXTENSION_CPPTOC_H_
+#define CEF_LIBCEF_DLL_CPPTOC_EXTENSION_CPPTOC_H_
+#pragma once
+
+#if !defined(BUILDING_CEF_SHARED)
+#error This file can be included DLL-side only
+#endif
+
+#include "include/capi/cef_extension_capi.h"
+#include "include/capi/cef_extension_handler_capi.h"
+#include "include/capi/cef_request_context_capi.h"
+#include "include/cef_extension.h"
+#include "include/cef_extension_handler.h"
+#include "include/cef_request_context.h"
+#include "libcef_dll/cpptoc/cpptoc_ref_counted.h"
+
+// Wrap a C++ class with a C structure.
+// This class may be instantiated and accessed DLL-side only.
+class CefExtensionCppToC : public CefCppToCRefCounted<CefExtensionCppToC,
+                                                      CefExtension,
+                                                      cef_extension_t> {
+ public:
+  CefExtensionCppToC();
+};
+
+#endif  // CEF_LIBCEF_DLL_CPPTOC_EXTENSION_CPPTOC_H_