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_background_host.h

@@ -0,0 +1,48 @@
+// 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_BACKGROUND_HOST_H_
+#define CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_BACKGROUND_HOST_H_
+
+#include <memory>
+
+#include "base/callback_forward.h"
+#include "base/macros.h"
+#include "extensions/browser/extension_host.h"
+
+class CefBrowserHostImpl;
+
+namespace content {
+class WebContents;
+}  // namespace content
+
+namespace extensions {
+
+// The ExtensionHost for a background page. This is a thin wrapper around the
+// ExtensionHost base class to support CEF-specific constructor. Object lifespan
+// is managed by ProcessManager.
+class CefExtensionBackgroundHost : public ExtensionHost {
+ public:
+  CefExtensionBackgroundHost(CefBrowserHostImpl* browser,
+                             base::OnceClosure deleted_callback,
+                             const Extension* extension,
+                             content::BrowserContext* browser_context,
+                             content::WebContents* host_contents,
+                             const GURL& url,
+                             ViewType host_type);
+  ~CefExtensionBackgroundHost() override;
+
+  // content::WebContentsDelegate methods:
+  bool ShouldTransferNavigation(bool is_main_frame_navigation) override;
+
+ private:
+  // Callback that will be executed on host deletion.
+  base::OnceClosure deleted_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(CefExtensionBackgroundHost);
+};
+
+}  // namespace extensions
+
+#endif  // CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_BACKGROUND_HOST_H_