Issue #1916 - Part 1: Convert flags passed to ReflowChild, FinishReflowChild, etc into an enum class.

Backported from Mozilla bug 1571250.

layout/mathml/nsMathMLmrootFrame.cpp

@@ -333,7 +333,7 @@ nsMathMLmrootFrame::Reflow(nsPresContext*          aPresContext,
     (indexRaisedAscent + indexSize.BlockStartAscent() - bmIndex.ascent);
   FinishReflowChild(indexFrame, aPresContext, indexSize, nullptr,
                     MirrorIfRTL(aDesiredSize.Width(), indexSize.Width(), dx),
-                    dy, 0);
+                    dy, ReflowChildFlags::Default);
 
   // place the radical symbol and the radical bar
   dx = dxSqr;
@@ -347,8 +347,8 @@ nsMathMLmrootFrame::Reflow(nsPresContext*          aPresContext,
   // place the base
   dy = aDesiredSize.BlockStartAscent() - baseSize.BlockStartAscent();
   FinishReflowChild(baseFrame, aPresContext, baseSize, nullptr,
-                    MirrorIfRTL(aDesiredSize.Width(), baseSize.Width(), dx),
-                    dy, 0);
+                    MirrorIfRTL(aDesiredSize.Width(), baseSize.Width(), dx), dy,
+                    ReflowChildFlags::Default);
 
   mReference.x = 0;
   mReference.y = aDesiredSize.BlockStartAscent();