[css-grid] Fix a dangling reference

The code was making a reference to an attribute of a temporary object created by gridTrackSize().

BUG=641982

Review-Url: https://codereview.chromium.org/2325603003
Cr-Commit-Position: refs/heads/master@{#417309}

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

@@ -2080,7 +2080,7 @@ LayoutUnit LayoutGrid::assumedRowsSizeForOrthogonalChild(const LayoutBox& child,
     bool gridAreaIsIndefinite = false;
     LayoutUnit containingBlockAvailableSize = containingBlockLogicalHeightForContent(ExcludeMarginBorderPadding);
     for (auto trackPosition : span) {
-        const GridLength& maxTrackSize = gridTrackSize(ForRows, trackPosition, sizingOperation).maxTrackBreadth();
+        GridLength maxTrackSize = gridTrackSize(ForRows, trackPosition, sizingOperation).maxTrackBreadth();
         if (maxTrackSize.isContentSized() || maxTrackSize.isFlex())
             gridAreaIsIndefinite = true;
         else