[SPv1] Always set a CompositorElementId on main graphics layers; use PaintLayer id.

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

Index: third_party/WebKit/Source/platform/graphics/CompositorElementId.h
diff --git a/third_party/WebKit/Source/platform/graphics/CompositorElementId.h b/third_party/WebKit/Source/platform/graphics/CompositorElementId.h
index 8ff066b4aba425245c0c67cd22ac20100975777b..6168a6a7927955fb1edfa5e9fc07c22de9e66026 100644
--- a/third_party/WebKit/Source/platform/graphics/CompositorElementId.h
+++ b/third_party/WebKit/Source/platform/graphics/CompositorElementId.h
@@ -17,19 +17,29 @@ const int kCompositorNamespaceBitCount = 3;
 
 enum class CompositorElementIdNamespace {
   kPrimary,
+  kRootScroll,
   kScroll,
+  kScrollbar,
+  kScrollState,
   kViewport,
   kLinkHighlight,
-  kScrollbar,
   // A sentinel to indicate the maximum representable namespace id
   // (the maximum is one less than this value).
   kMaxRepresentableNamespaceId = 1 << kCompositorNamespaceBitCount
 };
 
 using CompositorElementId = cc::ElementId;
-using DOMNodeId = uint64_t;
+using PaintLayerId = uint64_t;
 using ScrollbarId = uint64_t;
+using DOMNodeId = uint64_t;
+
+CompositorElementId PLATFORM_EXPORT
+    CompositorElementIdFromPaintLayerId(PaintLayerId,
+                                        CompositorElementIdNamespace);
 
+// This method should only be used for "special" layers that are not allocated
+// during the normal lifecycle. Examples include VisualViewport,
+// root scrolling (when rootLayerScrollingEnabled is off), and LinkHighlight.
 CompositorElementId PLATFORM_EXPORT
     CompositorElementIdFromDOMNodeId(DOMNodeId, CompositorElementIdNamespace);