Set a direct compositing reason for 3D transform & will-change property tree nodes

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

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
index a0a17d3962ef13e7d138575fc7b25be600a0f5ca..7c9237d6f295a9db8ba8c47edd437e860a7525fb 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
@@ -9,6 +9,7 @@
 #include "core/frame/Settings.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
+#include "core/layout/compositing/CompositingReasonFinder.h"
 #include "core/layout/svg/LayoutSVGRoot.h"
 #include "core/paint/FindPropertiesNeedingUpdate.h"
 #include "core/paint/ObjectPaintProperties.h"
@@ -319,6 +320,10 @@ void PaintPropertyTreeBuilder::updateTransform(
   if (object.needsPaintPropertyUpdate() || context.forceSubtreeUpdate) {
     const ComputedStyle& style = object.styleRef();
     if (object.isBox() && (style.hasTransform() || style.preserves3D())) {
+      CompositingReasons compositingReasons = CompositingReasonNone;
+      if (CompositingReasonFinder::requiresCompositingForTransform(object))
+        compositingReasons |= CompositingReason3DTransform;
+
       TransformationMatrix matrix;
       style.applyTransform(
           matrix, toLayoutBox(object).size(),
@@ -338,7 +343,8 @@ void PaintPropertyTreeBuilder::updateTransform(
       context.forceSubtreeUpdate |= properties.updateTransform(
           context.current.transform, matrix,
           transformOrigin(toLayoutBox(object)),
-          context.current.shouldFlattenInheritedTransform, renderingContextID);
+          context.current.shouldFlattenInheritedTransform, renderingContextID,
+          compositingReasons);
     } else {
       if (auto* properties = object.getMutableForPainting().paintProperties())
         context.forceSubtreeUpdate |= properties->clearTransform();