Adapt and reland old position sticky implementation

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

Index: third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h b/third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h
index d88539459d0f918d54338e92fbbc210063103d2e..d9220b257ef3e7be2b02ee1fb7cbcb6f05fd3c6e 100644
--- a/third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h
@@ -497,7 +497,13 @@ public:
     void updateDescendantDependentCompositingInputs(const DescendantDependentCompositingInputs&);
     void didUpdateCompositingInputs();
 
-    const AncestorDependentCompositingInputs& ancestorDependentCompositingInputs() const { ASSERT(!m_needsAncestorDependentCompositingInputsUpdate); return m_ancestorDependentCompositingInputs; }
+    const AncestorDependentCompositingInputs& ancestorDependentCompositingInputs() const
+    {
+        // TODO(flackr): When we process scroll updates after compositing updates we won't be querying ancestor
+        // dependent compositing inputs while they're still needed: http://crbug.com/420741.
+        ASSERT(isAllowedToQueryCompositingState() || !m_needsAncestorDependentCompositingInputsUpdate);
+        return m_ancestorDependentCompositingInputs;
+    }
     const DescendantDependentCompositingInputs& descendantDependentCompositingInputs() const { ASSERT(!m_needsDescendantDependentCompositingInputsUpdate); return m_descendantDependentCompositingInputs; }
 
     IntRect clippedAbsoluteBoundingBox() const { return ancestorDependentCompositingInputs().clippedAbsoluteBoundingBox; }