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_host_delegate.h

@@ -0,0 +1,45 @@
+// Copyright 2017 the Chromium Embedded Framework Authors. Portions copyright
+// 2014 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 LIBCEF_BROWSER_EXTENSIONS_EXTENSION_HOST_DELEGATE_H_
+#define LIBCEF_BROWSER_EXTENSIONS_EXTENSION_HOST_DELEGATE_H_
+
+#include "base/macros.h"
+#include "extensions/browser/extension_host_delegate.h"
+
+class CefBrowserHostImpl;
+
+namespace extensions {
+
+class CefExtensionHostDelegate : public ExtensionHostDelegate {
+ public:
+  explicit CefExtensionHostDelegate(CefBrowserHostImpl* browser);
+  ~CefExtensionHostDelegate() override;
+
+  // ExtensionHostDelegate implementation.
+  void OnExtensionHostCreated(content::WebContents* web_contents) override;
+  void OnRenderViewCreatedForBackgroundPage(ExtensionHost* host) override;
+  content::JavaScriptDialogManager* GetJavaScriptDialogManager() override;
+  void CreateTab(content::WebContents* web_contents,
+                 const std::string& extension_id,
+                 WindowOpenDisposition disposition,
+                 const gfx::Rect& initial_rect,
+                 bool user_gesture) override;
+  void ProcessMediaAccessRequest(content::WebContents* web_contents,
+                                 const content::MediaStreamRequest& request,
+                                 const content::MediaResponseCallback& callback,
+                                 const Extension* extension) override;
+  bool CheckMediaAccessPermission(content::WebContents* web_contents,
+                                  const GURL& security_origin,
+                                  content::MediaStreamType type,
+                                  const Extension* extension) override;
+  ExtensionHostQueue* GetExtensionHostQueue() const override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(CefExtensionHostDelegate);
+};
+
+}  // namespace extensions
+
+#endif  // LIBCEF_BROWSER_EXTENSIONS_EXTENSION_HOST_DELEGATE_H_