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

third_party/WebKit/Source/core/paint/PaintLayer.h

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index c8675f4de9d0cc91f2b59eeeafe1836a1ab7ea72..f1630b4294ea23d70b815dd2634a7155e15b5855 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -74,6 +74,8 @@ class HitTestingTransformState;
 class PaintLayerCompositor;
 class TransformationMatrix;
 
+using PaintLayerId = uint64_t;
+
 enum IncludeSelfOrNot { kIncludeSelf, kExcludeSelf };
 
 enum CompositingQueryMode {
@@ -1023,6 +1025,9 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
   void EndShouldKeepAliveAllClientsRecursive();
 #endif
 
+  // An id for this PaintLayer that is unique for the lifetime of the WebView.
+  PaintLayerId UniqueId() const { return unique_id_; }
+
  private:
   void SetNeedsCompositingInputsUpdateInternal();
 
@@ -1270,6 +1275,8 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
 
   std::unique_ptr<PaintLayerRareData> rare_data_;
 
+  PaintLayerId unique_id_;
+
   FRIEND_TEST_ALL_PREFIXES(PaintLayerTest,
                            DescendantDependentFlagsStopsAtThrottledFrames);
   FRIEND_TEST_ALL_PREFIXES(PaintLayerTest,