make track size const and improve comments

Author: intergalacticspacehighway

yoga/algorithm/grid/GridTrack.h

@@ -11,11 +11,11 @@
 
 namespace facebook::yoga {
 
-// Represents a resolved grid track size during layout computation. 
-// Includes variables used by the track sizing algorithm.
 // https://www.w3.org/TR/css-grid-1/#algo-track-sizing
 struct GridTrack {
-  GridTrackSize trackSize;
+  // Sizing functions for this track, immutable after construction
+  const GridTrackSize trackSize;
+  // Mutable state used by the track sizing algorithm
   float baseSize = 0.0f;
   float growthLimit = 0.0f;
   bool infinitelyGrowable = false;