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 561439aa9dfa64592c5f8fa0ca120a491e05caab..b9fc52f1614e8e8d8ba7aec0de6906023f0fa481 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -2277,6 +2277,13 @@ 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;
 }