[SPv2] Introduce an option to output the GraphicsLayer tree as a flat layer list.

third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h

Index: third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h
diff --git a/third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h b/third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h
index 93b65e05250416e83a556f959b963757de860a56..501c131cbddaef6e874ecc27c2de414f5c0893ab 100644
--- a/third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h
+++ b/third_party/WebKit/Source/platform/graphics/GraphicsLayerClient.h
@@ -58,6 +58,9 @@ enum {
   LayerTreeIncludesRootLayer = 1 << 3,
   LayerTreeIncludesClipAndScrollParents = 1 << 4,
   LayerTreeIncludesCompositingReasons = 1 << 5,
+  OutputChildrenAsLayerList =
+      1
+      << 6,  // Outputs all children of the given layer as a layer list, in paint order.
 };
 typedef unsigned LayerTreeFlags;