Set layer element id when building layers in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.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 PropertyTreeState_h
 #define PropertyTreeState_h
 
 #include "platform/graphics/paint/ClipPaintPropertyNode.h"
 #include "platform/graphics/paint/EffectPaintPropertyNode.h"
 #include "platform/graphics/paint/ScrollPaintPropertyNode.h"
@@ skipping 50 matching lines @@
   }
 
   const ScrollPaintPropertyNode* scroll() const {
     DCHECK(!m_scroll || !m_scroll->hasOneRef());
     return m_scroll.get();
   }
   void setScroll(RefPtr<const ScrollPaintPropertyNode> node) {
     m_scroll = std::move(node);
   }
 
+  // Returns the compositor element id, if any, for this property state. If none
+  // of the scroll, effect or transform nodes for this state have a compositor
+  // element id then a default instance is returned.
+  const CompositorElementId compositorElementId() const;

[wkorman, 2017/01/20 00:56:38]

We could put this logic in PaintArtifactCompositor as a static method but seemed
cleaner this way, open to thoughts though.

+
   enum InnermostNode {
     None,  // None means that all nodes are their root values
     Transform,
     Clip,
     Effect,
   };
 
   // There is always a well-defined order in which the transform, clip
   // and effects of a PropertyTreeState apply. This method returns which
   // of them applies first to content drawn with this PropertyTreeState.
@@ skipping 54 matching lines @@
       : m_properties(properties) {}
   const PropertyTreeState* next();
 
  private:
   PropertyTreeState m_properties;
 };
 
 }  // namespace blink
 
 #endif  // PropertyTreeState_h