Fix border radius on composited children.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index 894f7d8e93f14ad42f8ad43f5a2f0513a35b4157..cfe134a8b9bd8c820b2ee6657e6b5f0d927994cc 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -2257,6 +2257,13 @@ bool LayoutBox::paintedOutputOfObjectHasNoEffectRegardlessOfSize() const {
       return false;
   }
 
+  // If the box paints into its own backing, we can assume that it's painting
+  // may have some effect. For example, honoring the border-radius clip on
+  // a composited child paints into a mask for an otherwise non-painting
+  // element, because children of that element will require the mask.
+  if (hasLayer() && layer()->compositingState() == PaintsIntoOwnBacking)
+    return false;
+
   return true;
 }