Bug 548964 backing out due to reftest failure only on Win Opt (433700) a=orange

gfx/thebes/gfxDWriteFonts.cpp

@@ -199,7 +199,11 @@ gfxDWriteFont::ComputeMetrics()
         mFontFace->ReleaseFontTable(tableContext);
     }
 
-    mMetrics.internalLeading = NS_MAX(mMetrics.maxHeight - mMetrics.emHeight, 0.0);
+    mMetrics.internalLeading = 
+        ceil(((gfxFloat)(fontMetrics.ascent + 
+                    fontMetrics.descent - 
+                    fontMetrics.designUnitsPerEm) / 
+                    fontMetrics.designUnitsPerEm) * mAdjustedSize);
     mMetrics.externalLeading = 
         ceil(((gfxFloat)fontMetrics.lineGap /
                    fontMetrics.designUnitsPerEm) * mAdjustedSize);