[DevTools] Use WebLayerTreeView::layerTreeId() for instrumentation.

public/platform/WebLayerTreeView.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 123 matching lines @@
     virtual void clearViewportLayers() { }
 
     // Used to update the active selection bounds.
     // FIXME: Remove this overload when downstream consumers have been updated to use WebSelection, crbug.com/466672.
     virtual void registerSelection(const WebSelectionBound& start, const WebSelectionBound& end) { }
     virtual void registerSelection(const WebSelection&) { }
     virtual void clearSelection() { }
 
     // Debugging / dangerous ---------------------------------------------
 
+    virtual int layerTreeId() const { return 0; }
+
     // Toggles the FPS counter in the HUD layer
     virtual void setShowFPSCounter(bool) { }
 
     // Toggles the paint rects in the HUD layer
     virtual void setShowPaintRects(bool) { }
 
     // Toggles the debug borders on layers
     virtual void setShowDebugBorders(bool) { }
 
     // Toggles continuous painting
     virtual void setContinuousPaintingEnabled(bool) { }
 
     // Toggles scroll bottleneck rects on the HUD layer
     virtual void setShowScrollBottleneckRects(bool) { }
 };
 
 } // namespace blink
 
 #endif // WebLayerTreeView_h