[css-grid] Return cached track sizes by reference

Currently GridTrackSizingAlgorithmStrategy::GetCachedGridTrackSize and
GridTrack::CachedTrackSize don't return the GridTrackSize by reference.

This means that some code like

  const GridLength& length = track.CachedTrackSize().MaxTrackBreadth();
  DCHECK(length.IsFlex());

has a stack-use-after-scope issue, since GridTrackSize::MaxTrackBreadth
returns a GridLength by reference, which will point to the temporary
object returned by CachedTrackSize.

To avoid this problem, this patch makes GetCachedGridTrackSize and
CachedTrackSize return a const reference instead of a temporary copy.

Change-Id: I3e43bf88a475b30747a101195705a22d93cc52a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919140Reviewed-by: Sergio Villar <svillar@igalia.com>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#715665}

third_party/blink/renderer/core/layout/grid_track_sizing_algorithm.h

@@ -66,7 +66,7 @@ class GridTrack {
   }
   void SetGrowthLimitCap(base::Optional<LayoutUnit>);
 
-  const GridTrackSize CachedTrackSize() const {
+  const GridTrackSize& CachedTrackSize() const {
     DCHECK(cached_track_size_.has_value());
     return cached_track_size_.value();
   }
@@ -325,8 +325,9 @@ class GridTrackSizingAlgorithmStrategy {
     return algorithm_.AvailableSpace();
   }
 
-  GridTrackSize GetCachedGridTrackSize(GridTrackSizingDirection direction,
-                                       size_t translated_index) const {
+  const GridTrackSize& GetCachedGridTrackSize(
+      GridTrackSizingDirection direction,
+      size_t translated_index) const {
     return algorithm_.Tracks(direction)[translated_index].CachedTrackSize();
   }