Fix cc scrollbar layer issues with initialization, and use element ids throughout.

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"
 #include "platform/wtf/HashSet.h"
 #include "platform/wtf/HashTraits.h"
 
 namespace blink {
 
-enum class CompositorSubElementId {
+const int kCompositorNamespaceBitCount = 3;
+
+enum class CompositorElementIdNamespace {
   kPrimary,
   kScroll,
   kViewport,
   kLinkHighlight,
-  // A sentinel to indicate how many enum values there are.
-  kNumSubElementTypes
+  kScrollbar,
+  // A sentinel to indicate the maximum representable namespace id
+  // (the maximum is one less than this value).
+  kMaxRepresentableNamespaceId = 1 << kCompositorNamespaceBitCount
 };
 
 using CompositorElementId = cc::ElementId;
 using DOMNodeId = uint64_t;
+using ScrollbarId = uint64_t;
 
 CompositorElementId PLATFORM_EXPORT
-    CreateCompositorElementId(DOMNodeId, CompositorSubElementId);
+    CompositorElementIdFromDOMNodeId(DOMNodeId, CompositorElementIdNamespace);
+
+CompositorElementId PLATFORM_EXPORT
+    CompositorElementIdFromScrollbarId(ScrollbarId,
+                                       CompositorElementIdNamespace);
 
 // Note cc::ElementId has a hash function already implemented via
 // ElementIdHash::operator(). However for consistency's sake we choose to use
 // Blink's hash functions with Blink specific data structures.
 struct CompositorElementIdHash {
   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;
 };
 
-DOMNodeId PLATFORM_EXPORT DomNodeIdFromCompositorElementId(CompositorElementId);
-CompositorSubElementId PLATFORM_EXPORT
-    SubElementIdFromCompositorElementId(CompositorElementId);
+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