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/browser/extension_impl.h

@@ -0,0 +1,58 @@
+// 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.
+
+#ifndef CEF_LIBCEF_BROWSER_EXTENSION_IMPL_H_
+#define CEF_LIBCEF_BROWSER_EXTENSION_IMPL_H_
+#pragma once
+
+#include <memory>
+
+#include "include/cef_extension.h"
+#include "include/cef_extension_handler.h"
+#include "include/cef_request_context.h"
+
+namespace extensions {
+class Extension;
+};
+
+// CefNavigationEntry implementation
+class CefExtensionImpl : public CefExtension {
+ public:
+  CefExtensionImpl(const extensions::Extension* extension,
+                   CefRequestContext* loader_context,
+                   CefRefPtr<CefExtensionHandler> handler);
+
+  // CefExtension methods.
+  CefString GetIdentifier() override;
+  CefString GetPath() override;
+  CefRefPtr<CefDictionaryValue> GetManifest() override;
+  bool IsSame(CefRefPtr<CefExtension> that) override;
+  CefRefPtr<CefExtensionHandler> GetHandler() override;
+  CefRefPtr<CefRequestContext> GetLoaderContext() override;
+  bool IsLoaded() override;
+  void Unload() override;
+
+  void OnExtensionLoaded();
+  void OnExtensionUnloaded();
+
+  // Use this instead of the GetLoaderContext version during
+  // CefRequestContext destruction.
+  CefRequestContext* loader_context() const { return loader_context_; }
+
+ private:
+  CefString id_;
+  CefString path_;
+  CefRefPtr<CefDictionaryValue> manifest_;
+
+  CefRequestContext* loader_context_;
+  CefRefPtr<CefExtensionHandler> handler_;
+
+  // Only accessed on the UI thread.
+  bool unloaded_ = false;
+
+  IMPLEMENT_REFCOUNTING(CefExtensionImpl);
+  DISALLOW_COPY_AND_ASSIGN(CefExtensionImpl);
+};
+
+#endif  // CEF_LIBCEF_BROWSER_EXTENSION_IMPL_H_