Avoid element id collisions with the visual viewport

third_party/WebKit/Source/platform/graphics/CompositorElementId.h

 // Copyright 2016 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 CompositorElementId_h
 #define CompositorElementId_h
 
 #include "cc/animation/element_id.h"
 #include "platform/PlatformExport.h"
 
 namespace blink {
 
-enum class CompositorSubElementId { Primary, Scroll, LinkHighlight };
+enum class CompositorSubElementId { Primary, Scroll, Viewport, LinkHighlight };
 
 using CompositorElementId = cc::ElementId;
 
 CompositorElementId PLATFORM_EXPORT
 createCompositorElementId(int domNodeId, CompositorSubElementId);
 
 }  // namespace blink
 
 #endif  // CompositorElementId_h