Add GetWebContents to ShellDelegate.

ash/shell/shell_delegate_impl.cc

 // Copyright (c) 2012 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 "ash/shell/shell_delegate_impl.h"
 
 #include "ash/common/accessibility_delegate.h"
 #include "ash/common/default_accessibility_delegate.h"
 #include "ash/common/gpu_support_stub.h"
 #include "ash/common/palette_delegate.h"
@@ skipping 157 matching lines @@
 bool ShellDelegateImpl::IsTouchscreenEnabledInPrefs(
     bool use_local_state) const {
   return true;
 }
 
 void ShellDelegateImpl::SetTouchscreenEnabledInPrefs(bool enabled,
                                                      bool use_local_state) {}
 
 void ShellDelegateImpl::UpdateTouchscreenStatusFromPrefs() {}
 
+content::WebContents* ShellDelegateImpl::GetWebContents() {
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
 }  // namespace shell
 }  // namespace ash