[css-grid] mimax(auto, <flex>) should be serialized as <flex>

third_party/WebKit/Source/core/style/GridLength.h

Index: third_party/WebKit/Source/core/style/GridLength.h
diff --git a/third_party/WebKit/Source/core/style/GridLength.h b/third_party/WebKit/Source/core/style/GridLength.h
index 67f4f6808c4f1c379fb3f7ddcf60cd03b199565f..286c35968cddcfe69a27f5ac5be6a5c2b76ca630 100644
--- a/third_party/WebKit/Source/core/style/GridLength.h
+++ b/third_party/WebKit/Source/core/style/GridLength.h
@@ -76,6 +76,8 @@ class GridLength {
             m_length.isMaxContent());
   }
 
+  bool isAuto() const { return m_type == LengthType && m_length.isAuto(); }
+
  private:
   // Ideally we would put the 2 following fields in a union, but Length has a
   // constructor, a destructor and a copy assignment which isn't allowed.