[RootLayerScrolls] Annotate non-fast-scroll regions to correct layer

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 168383e4077bc5da12b3a5d8b84841d30bb4efcc..d891178e1a9a6d36b791cd61f37a209754dde810 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -47,6 +47,7 @@
 #include "core/CSSPropertyNames.h"
 #include "core/HTMLNames.h"
 #include "core/css/PseudoStyleRequest.h"
+#include "core/dom/ClientRectList.h"
 #include "core/dom/Document.h"
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/frame/FrameView.h"
@@ -3198,6 +3199,18 @@ void PaintLayer::clearNeedsRepaintRecursively() {
   m_needsRepaint = false;
 }
 
+ClientRectList* PaintLayer::nonFastScrollableRects() const {
+  // This bypasses the isAllowedToQueryCompositingState() check to enabled the
+  // optimization from https://codereview.chromium.org/1256043007
+  CompositedLayerMapping* clm =
+      m_rareData ? m_rareData->compositedLayerMapping.get() : nullptr;
+  if (!clm || !clm->scrollingContentsLayer())
+    return ClientRectList::create();
+  return ClientRectList::create(clm->scrollingContentsLayer()
+                                    ->platformLayer()
+                                    ->nonFastScrollableRegion());
+}
+
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS
 void PaintLayer::endShouldKeepAliveAllClientsRecursive() {
   for (PaintLayer* child = firstChild(); child; child = child->nextSibling())