[SPv2] Decomposite otherwise-compositable animations that paint nothing.

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 "wtf/HashFunctions.h"
+#include "wtf/HashSet.h"
+#include "wtf/HashTraits.h"
 
 namespace blink {
 
 enum class CompositorSubElementId { Primary, Scroll, Viewport, LinkHighlight };
 
 using CompositorElementId = cc::ElementId;
 
 CompositorElementId PLATFORM_EXPORT
 createCompositorElementId(int domNodeId, CompositorSubElementId);
 
+// 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 hash(const CompositorElementId& p) {
+    return WTF::hashInts(p.primaryId, p.secondaryId);
+  }
+  static bool equal(const CompositorElementId& a,
+                    const CompositorElementId& b) {
+    return a.primaryId == b.primaryId && a.secondaryId == b.secondaryId;
+  }
+  static const bool safeToCompareToEmptyOrDeleted = true;
+};
+
+struct CompositorElementIdHashTraits
+    : WTF::GenericHashTraits<CompositorElementId> {
+  static CompositorElementId emptyValue() { return CompositorElementId(); }
+  static void constructDeletedValue(CompositorElementId& slot, bool) {
+    slot = CompositorElementId(-1, -1);
+  }
+  static bool isDeletedValue(CompositorElementId value) {
+    return value == CompositorElementId(-1, -1);
+  }
+};
+
+using CompositorElementIdSet = HashSet<CompositorElementId,
+                                       CompositorElementIdHash,
+                                       CompositorElementIdHashTraits>;
+
 }  // namespace blink
 
 #endif  // CompositorElementId_h