[css-grid] Fix behavior of flexible track breadths

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

Index: third_party/WebKit/Source/core/layout/LayoutGrid.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
index 3b90d11e1fb559c8dd4177a2688311182ed79a31..efda3881637b957a46a721d1a50d5fc864503497 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
@@ -745,6 +745,11 @@ GridTrackSize LayoutGrid::gridTrackSize(GridTrackSizingDirection direction, size
         }
     }
 
+    // Flex sizes are invalid as a min sizing function. However we still can have a flexible |minTrackBreadth|
+    // if the track had a flex size directly (e.g. "1fr"), the spec says that in this case it implies an automatic minimum.
+    if (minTrackBreadth.isFlex())
+        minTrackBreadth = Length(Auto);
+
     return GridTrackSize(minTrackBreadth, maxTrackBreadth);
 }