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.cc

@@ -0,0 +1,46 @@
+// 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.
+
+#include "libcef/browser/extensions/extension_background_host.h"
+
+#include "libcef/browser/browser_host_impl.h"
+#include "libcef/browser/extensions/extension_host_delegate.h"
+
+#include "base/callback.h"
+
+namespace extensions {
+
+CefExtensionBackgroundHost::CefExtensionBackgroundHost(
+    CefBrowserHostImpl* browser,
+    base::OnceClosure deleted_callback,
+    const Extension* extension,
+    content::BrowserContext* browser_context,
+    content::WebContents* host_contents,
+    const GURL& url,
+    ViewType host_type)
+    : ExtensionHost(new CefExtensionHostDelegate(browser),
+                    extension,
+                    browser_context,
+                    host_contents,
+                    url,
+                    host_type),
+      deleted_callback_(std::move(deleted_callback)) {
+  DCHECK(!deleted_callback_.is_null());
+
+  // Only used for background pages.
+  DCHECK(host_type == VIEW_TYPE_EXTENSION_BACKGROUND_PAGE);
+}
+
+CefExtensionBackgroundHost::~CefExtensionBackgroundHost() {
+  std::move(deleted_callback_).Run();
+}
+
+bool CefExtensionBackgroundHost::ShouldTransferNavigation(
+    bool is_main_frame_navigation) {
+  // Block navigations that cause the main frame to navigate to non-extension
+  // content (i.e. to web content).
+  return !is_main_frame_navigation;
+}
+
+}  // namespace extensions