Return '' for overflow if overflow-x and overflow-y are different.

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

Index: third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
diff --git a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
index 5394b40317a46dc87b92dbe6c144480c61213132..266efb46001f63af3c6cf059d6e68a009948cdb4 100644
--- a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
+++ b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
@@ -2682,8 +2682,9 @@ const CSSValue* ComputedStyleCSSValueMapping::get(
     case CSSPropertyOutlineWidth:
       return zoomAdjustedPixelValue(style.outlineWidth(), style);
     case CSSPropertyOverflow:
-      return CSSIdentifierValue::create(
-          max(style.overflowX(), style.overflowY()));
+      if (style.overflowX() == style.overflowY())
+        return CSSIdentifierValue::create(style.overflowX());
+      return nullptr;
     case CSSPropertyOverflowAnchor:
       return CSSIdentifierValue::create(style.overflowAnchor());
     case CSSPropertyOverflowWrap: