[CSS Grid Layout] Percentages of indefinite sizes should compute to "auto"

Source/core/rendering/style/GridTrackSize.h

Index: Source/core/rendering/style/GridTrackSize.h
diff --git a/Source/core/rendering/style/GridTrackSize.h b/Source/core/rendering/style/GridTrackSize.h
index 584b47a4024f4f7bf4923ce7612f20c60e525d9b..535394e9926402860e9d56bba9a78cb1913e5a5b 100644
--- a/Source/core/rendering/style/GridTrackSize.h
+++ b/Source/core/rendering/style/GridTrackSize.h
@@ -120,6 +120,8 @@ public:
 
     bool isContentSized() const { return m_minTrackBreadth.isContentSized() || m_maxTrackBreadth.isContentSized(); }
 
+    bool isPercentage() const { return m_type == LengthTrackSizing && length().isLength() && length().length().isPercent(); }
+
     bool operator==(const GridTrackSize& other) const
     {
         return m_type == other.m_type && m_minTrackBreadth == other.m_minTrackBreadth && m_maxTrackBreadth == other.m_maxTrackBreadth;