Implement will-change-based creation of layers, stacking contexts, and containing blocks

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 0002195a36b4fa9f855323e59137556d1669d00b..7755f81b988691a05226110fc5a4350d0a0cd76d 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -101,6 +101,9 @@ CompositingReasons CompositingReasonFinder::directReasons(const RenderLayer* lay
     if (requiresCompositingForOutOfFlowClipping(layer))
         directReasons |= CompositingReasonOutOfFlowClipping;
 
+    if (requiresCompositingForWillChange(renderer))
+        directReasons |= CompositingReasonWillChange;
+
     return directReasons;
 }
 
@@ -258,6 +261,11 @@ bool CompositingReasonFinder::requiresCompositingForOutOfFlowClipping(const Rend
     return m_renderView.compositorDrivenAcceleratedScrollingEnabled() && layer->isUnclippedDescendant();
 }
 
+bool CompositingReasonFinder::requiresCompositingForWillChange(const RenderObject* renderer) const
+{
+    return renderer->style()->hasWillChangeCompositingHint();
+}
+
 bool CompositingReasonFinder::isViewportConstrainedFixedOrStickyLayer(const RenderLayer* layer)
 {
     if (layer->renderer()->isStickyPositioned())