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.cc

@@ -0,0 +1,97 @@
+// 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_view_host.h"
+
+#include "libcef/browser/browser_platform_delegate.h"
+#include "libcef/browser/extensions/extension_host_delegate.h"
+
+#include "content/public/browser/notification_source.h"
+#include "content/public/browser/web_contents.h"
+#include "extensions/browser/extension_system.h"
+#include "extensions/browser/notification_types.h"
+#include "extensions/browser/runtime_data.h"
+#include "third_party/WebKit/public/platform/WebGestureEvent.h"
+
+using content::NativeWebKeyboardEvent;
+using content::OpenURLParams;
+using content::WebContents;
+using content::WebContentsObserver;
+
+namespace extensions {
+
+CefExtensionViewHost::CefExtensionViewHost(
+    CefBrowserHostImpl* browser,
+    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) {
+  // Only used for dialogs and popups.
+  DCHECK(host_type == VIEW_TYPE_EXTENSION_DIALOG ||
+         host_type == VIEW_TYPE_EXTENSION_POPUP);
+}
+
+CefExtensionViewHost::~CefExtensionViewHost() {}
+
+void CefExtensionViewHost::OnDidStopFirstLoad() {
+  // Nothing to do here, but don't call the base class method.
+}
+
+void CefExtensionViewHost::LoadInitialURL() {
+  if (!ExtensionSystem::Get(browser_context())
+           ->runtime_data()
+           ->IsBackgroundPageReady(extension())) {
+    // Make sure the background page loads before any others.
+    registrar_.Add(this,
+                   extensions::NOTIFICATION_EXTENSION_BACKGROUND_PAGE_READY,
+                   content::Source<Extension>(extension()));
+    return;
+  }
+
+  ExtensionHost::LoadInitialURL();
+}
+
+bool CefExtensionViewHost::IsBackgroundPage() const {
+  return false;
+}
+
+bool CefExtensionViewHost::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;
+}
+
+bool CefExtensionViewHost::PreHandleGestureEvent(
+    content::WebContents* source,
+    const blink::WebGestureEvent& event) {
+  // Disable pinch zooming.
+  return blink::WebInputEvent::IsPinchGestureEventType(event.GetType());
+}
+
+WebContents* CefExtensionViewHost::GetVisibleWebContents() const {
+  if (extension_host_type() == VIEW_TYPE_EXTENSION_POPUP)
+    return host_contents();
+  return nullptr;
+}
+
+void CefExtensionViewHost::Observe(
+    int type,
+    const content::NotificationSource& source,
+    const content::NotificationDetails& details) {
+  DCHECK_EQ(type, extensions::NOTIFICATION_EXTENSION_BACKGROUND_PAGE_READY);
+  DCHECK(ExtensionSystem::Get(browser_context())
+             ->runtime_data()
+             ->IsBackgroundPageReady(extension()));
+  LoadInitialURL();
+}
+
+}  // namespace extensions