SPv2: Limit compositor element id application to a single layer.

third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
index 548f5810fa6111f0cee1cf398110bcfe9c9fa978..1b07e9698f63c204499401e4307b032767eb1e7f 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
@@ -38,25 +38,37 @@ bool IsAncestorOf(const PropertyNode* ancestor, const PropertyNode* child) {
   return child == ancestor;
 }
 
-const CompositorElementId PropertyTreeState::GetCompositorElementId() const {
+const CompositorElementId PropertyTreeState::GetCompositorElementId(
+    const CompositorElementIdSet& element_ids) const {
 // The effect or transform nodes could have a compositor element id. The order
 // doesn't matter as the element id should be the same on all that have a
 // non-default CompositorElementId.
+//
+// Note that PropertyTreeState acts as a context that accumulates state as we
+// traverse the tree building layers. This means that we could see a compositor
+// element id 'A' for a parent layer in conjunction with a compositor element id
+// 'B' for a child layer. To preserve uniqueness of element ids, then, we check
+// for presence in the |element_ids| set (which represents element ids already
+// previously attached to a layer). This is an interim step while we pursue
+// broader rework of animation subsystem noted in http://crbug.com/709137.
 #if DCHECK_IS_ON()
   CompositorElementId expected_element_id;
-  if (CompositorElementId actual_element_id =
-          Effect()->GetCompositorElementId()) {
-    expected_element_id = actual_element_id;
+  CompositorElementId effect_element_id = Effect()->GetCompositorElementId();
+  if (effect_element_id && !element_ids.Contains(effect_element_id)) {
+    expected_element_id = effect_element_id;
   }
-  if (CompositorElementId actual_element_id =
-          Transform()->GetCompositorElementId()) {
-    if (expected_element_id)
-      DCHECK_EQ(expected_element_id, actual_element_id);
+  CompositorElementId transform_element_id =
+      Transform()->GetCompositorElementId();
+  if (expected_element_id && transform_element_id &&
+      !element_ids.Contains(transform_element_id)) {
+    DCHECK_EQ(expected_element_id, transform_element_id);
   }
 #endif
-  if (Effect()->GetCompositorElementId())
+  if (Effect()->GetCompositorElementId() &&
+      !element_ids.Contains(Effect()->GetCompositorElementId()))
     return Effect()->GetCompositorElementId();
-  if (Transform()->GetCompositorElementId())
+  if (Transform()->GetCompositorElementId() &&
+      !element_ids.Contains(Transform()->GetCompositorElementId()))
     return Transform()->GetCompositorElementId();
   return CompositorElementId();
 }