Fix visual overflow for LayoutBoxes with numeric border image outsets

Source/core/style/BorderImageLength.h

Index: Source/core/style/BorderImageLength.h
diff --git a/Source/core/style/BorderImageLength.h b/Source/core/style/BorderImageLength.h
index 282037f329ae81bb182b775f1aea8e34e3ffd2a7..a52e788cd15d802f9d3d12c840aa25ad10f180a2 100644
--- a/Source/core/style/BorderImageLength.h
+++ b/Source/core/style/BorderImageLength.h
@@ -69,7 +69,11 @@ public:
 
     bool isZero() const
     {
-        return (isLength() && m_length.isZero()) || (isNumber() && m_number);
+        if (isLength())
+            return m_length.isZero();
+
+        ASSERT(isNumber());
+        return !m_number;
     }
 
 private: