Remove WebCompositorSupport::createDisplayItemList(), nothing uses it.

public/platform/WebCompositorSupport.h

Index: public/platform/WebCompositorSupport.h
diff --git a/public/platform/WebCompositorSupport.h b/public/platform/WebCompositorSupport.h
index f626c14fc21e14fd34cbe6e7ac4669339e59ed4e..7b6adaf953cad337f4aea5f763bae0ed750522a1 100644
--- a/public/platform/WebCompositorSupport.h
+++ b/public/platform/WebCompositorSupport.h
@@ -41,7 +41,6 @@ class WebCompositorAnimationPlayer;
 class WebCompositorAnimationTimeline;
 class WebContentLayer;
 class WebContentLayerClient;
-class WebDisplayItemList;
 class WebExternalTextureLayer;
 class WebExternalTextureLayerClient;
 class WebFilterAnimationCurve;
@@ -94,8 +93,6 @@ public:
 
     virtual WebCompositorAnimationTimeline* createAnimationTimeline() { return nullptr; }
 
-    virtual WebDisplayItemList* createDisplayItemList() { return nullptr; }
-
 protected:
     virtual ~WebCompositorSupport() { }
 };