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/extensions/extension_view_host.h

@@ -0,0 +1,64 @@
+// Copyright 2017 the Chromium Embedded Framework Authors. Portions copyright
+// 2013 The Chromium 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_EXTENSIONS_EXTENSION_VIEW_HOST_H_
+#define CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_VIEW_HOST_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "content/public/browser/notification_observer.h"
+#include "content/public/browser/notification_registrar.h"
+#include "extensions/browser/extension_host.h"
+
+class CefBrowserHostImpl;
+
+namespace content {
+class WebContents;
+}  // namespace content
+
+namespace extensions {
+
+// The ExtensionHost for an extension that backs a view in the browser UI. For
+// example, this could be an extension popup or dialog, but not a background
+// page. Object lifespan is managed by CefBrowserHostImpl. Based on
+// chrome/browser/extensions/extension_view_host.h.
+class CefExtensionViewHost : public ExtensionHost,
+                             public content::NotificationObserver {
+ public:
+  CefExtensionViewHost(CefBrowserHostImpl* browser,
+                       const Extension* extension,
+                       content::BrowserContext* browser_context,
+                       content::WebContents* host_contents,
+                       const GURL& url,
+                       ViewType host_type);
+  ~CefExtensionViewHost() override;
+
+  // ExtensionHost methods:
+  void OnDidStopFirstLoad() override;
+  void LoadInitialURL() override;
+  bool IsBackgroundPage() const override;
+
+  // content::WebContentsDelegate methods:
+  bool ShouldTransferNavigation(bool is_main_frame_navigation) override;
+  bool PreHandleGestureEvent(content::WebContents* source,
+                             const blink::WebGestureEvent& event) override;
+
+  // extensions::ExtensionFunctionDispatcher::Delegate methods:
+  content::WebContents* GetVisibleWebContents() const override;
+
+  // content::NotificationObserver methods:
+  void Observe(int type,
+               const content::NotificationSource& source,
+               const content::NotificationDetails& details) override;
+
+ private:
+  content::NotificationRegistrar registrar_;
+
+  DISALLOW_COPY_AND_ASSIGN(CefExtensionViewHost);
+};
+
+}  // namespace extensions
+
+#endif  // CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_VIEW_HOST_H_