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

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositorElementId_h
 #define CompositorElementId_h
 
 #include "cc/trees/element_id.h"
 #include "platform/PlatformExport.h"
 #include "platform/wtf/HashFunctions.h"
 #include "platform/wtf/HashSet.h"
 #include "platform/wtf/HashTraits.h"
 
 namespace blink {
 
 const int kCompositorNamespaceBitCount = 3;
 
 enum class CompositorElementIdNamespace {
   kPrimary,
+  kRootScroll,
   kScroll,
+  kScrollbar,
+  kScrollState,
   kViewport,
   kLinkHighlight,
-  kScrollbar,
+  kPrimaryCompositorProxy,
+  kScrollCompositorProxy,
   // 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 PaintLayerId = uint64_t;
+using ScrollbarId = uint64_t;
 using DOMNodeId = uint64_t;
-using ScrollbarId = 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,
+// or when CompositorProxies are involved.
+
+// Otherwise, CompositorElementIdFromPaintLayerId is preferred for performance
+// reasons (since computing a DOMNodeId requires a hash map lookup),
+// and future compatibility with multicol/pagination.
+CompositorElementId PLATFORM_EXPORT
     CompositorElementIdFromDOMNodeId(DOMNodeId, CompositorElementIdNamespace);
 
 CompositorElementId PLATFORM_EXPORT
     CompositorElementIdFromScrollbarId(ScrollbarId,
                                        CompositorElementIdNamespace);
 
 // Note cc::ElementId has a hash function already implemented via
 // ElementIdHash::operator(). However for consistency's sake we choose to use
 // Blink's hash functions with Blink specific data structures.
 struct CompositorElementIdHash {
@@ skipping 21 matching lines @@
   }
 };
 
 using CompositorElementIdSet = HashSet<CompositorElementId,
                                        CompositorElementIdHash,
                                        CompositorElementIdHashTraits>;
 
 }  // namespace blink
 
 #endif  // CompositorElementId_h