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

public/platform/WebLayerTreeView.h

Index: public/platform/WebLayerTreeView.h
diff --git a/public/platform/WebLayerTreeView.h b/public/platform/WebLayerTreeView.h
index afc35caa88353d17dd1f0829579c378f74bee743..c32660b4b47c04eed1fbedfad520a65f331e716d 100644
--- a/public/platform/WebLayerTreeView.h
+++ b/public/platform/WebLayerTreeView.h
@@ -141,6 +141,8 @@ public:
 
     // Debugging / dangerous ---------------------------------------------
 
+    virtual int layerTreeId() const { return 0; }
+
     // Toggles the FPS counter in the HUD layer
     virtual void setShowFPSCounter(bool) { }