Slimming Paint phase 2 compositing algorithm plumbing & skeleton display list API.

public/platform/WebContentLayerClient.h

Index: public/platform/WebContentLayerClient.h
diff --git a/public/platform/WebContentLayerClient.h b/public/platform/WebContentLayerClient.h
index abf25d77dcd1a817a4d3350c7d954628594be8cb..d81421e29a15efacf226a30a3d3772680968658f 100644
--- a/public/platform/WebContentLayerClient.h
+++ b/public/platform/WebContentLayerClient.h
@@ -32,6 +32,8 @@
 namespace blink {
 
 class WebDisplayItemList;
+class WebDisplayItemTransformTree;
+class WebDisplayList;
 struct WebRect;
 
 class BLINK_PLATFORM_EXPORT WebContentLayerClient {
@@ -69,6 +71,11 @@ public:
     // result includes data cached internally during painting.
     virtual size_t approximateUnsharedMemoryUsage() const { return 0; }
 
+    virtual const WebDisplayList* displayList() const { return 0; };
+
+    // This is only set on a root graphics layer. Alternatively, this could be set on the layer tree host.
+    virtual const WebDisplayItemTransformTree* transformTree() const { return 0; };
+
 protected:
     virtual ~WebContentLayerClient() { }
 };