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.cc

@@ -0,0 +1,63 @@
+// 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.
+
+#include "libcef/browser/extensions/extension_host_delegate.h"
+
+#include "libcef/browser/extensions/extensions_browser_client.h"
+
+#include "base/logging.h"
+
+namespace extensions {
+
+CefExtensionHostDelegate::CefExtensionHostDelegate(
+    CefBrowserHostImpl* browser) {}
+
+CefExtensionHostDelegate::~CefExtensionHostDelegate() {}
+
+void CefExtensionHostDelegate::OnExtensionHostCreated(
+    content::WebContents* web_contents) {}
+
+void CefExtensionHostDelegate::OnRenderViewCreatedForBackgroundPage(
+    ExtensionHost* host) {}
+
+content::JavaScriptDialogManager*
+CefExtensionHostDelegate::GetJavaScriptDialogManager() {
+  // Never routed here from CefBrowserHostImpl.
+  NOTREACHED();
+  return NULL;
+}
+
+void CefExtensionHostDelegate::CreateTab(content::WebContents* web_contents,
+                                         const std::string& extension_id,
+                                         WindowOpenDisposition disposition,
+                                         const gfx::Rect& initial_rect,
+                                         bool user_gesture) {
+  // TODO(cef): Add support for extensions opening popup windows.
+  NOTIMPLEMENTED();
+}
+
+void CefExtensionHostDelegate::ProcessMediaAccessRequest(
+    content::WebContents* web_contents,
+    const content::MediaStreamRequest& request,
+    const content::MediaResponseCallback& callback,
+    const Extension* extension) {
+  // Never routed here from CefBrowserHostImpl.
+  NOTREACHED();
+}
+
+bool CefExtensionHostDelegate::CheckMediaAccessPermission(
+    content::WebContents* web_contents,
+    const GURL& security_origin,
+    content::MediaStreamType type,
+    const Extension* extension) {
+  // Never routed here from CefBrowserHostImpl.
+  NOTREACHED();
+  return false;
+}
+
+ExtensionHostQueue* CefExtensionHostDelegate::GetExtensionHostQueue() const {
+  return CefExtensionsBrowserClient::Get()->GetExtensionHostQueue();
+}
+
+}  // namespace extensions