Stop retrieving scrolling element id from the CompositorElementId.

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/trees/element_id.h"
 #include "platform/PlatformExport.h"
 #include "platform/wtf/HashFunctions.h"
@@ skipping 33 matching lines @@
   static unsigned GetHash(const CompositorElementId& key) {
     return WTF::HashInt(static_cast<cc::ElementIdType>(key.id_));
   }
   static bool Equal(const CompositorElementId& a,
                     const CompositorElementId& b) {
     return a.id_ == b.id_;
   }
   static const bool safe_to_compare_to_empty_or_deleted = true;
 };
 
-uint64_t PLATFORM_EXPORT IdFromCompositorElementId(CompositorElementId);
 CompositorElementIdNamespace PLATFORM_EXPORT
     NamespaceFromCompositorElementId(CompositorElementId);
 
 struct CompositorElementIdHashTraits
     : WTF::GenericHashTraits<CompositorElementId> {
   static CompositorElementId EmptyValue() { return CompositorElementId(1); }
   static void ConstructDeletedValue(CompositorElementId& slot, bool) {
     slot = CompositorElementId();
   }
   static bool IsDeletedValue(CompositorElementId value) {
     return value == CompositorElementId();
   }
 };
 
 using CompositorElementIdSet = HashSet<CompositorElementId,
                                        CompositorElementIdHash,
                                        CompositorElementIdHashTraits>;
 
 }  // namespace blink
 
 #endif  // CompositorElementId_h