Use SetMEWToActualWidth when we set the MEW to the actual width. b=331684 r+sr=bzbarsky

layout/generic/nsObjectFrame.cpp

@@ -660,7 +660,7 @@ nsObjectFrame::GetDesiredSize(nsPresContext* aPresContext,
   aMetrics.ascent = aMetrics.height;
 
   if (aMetrics.mComputeMEW) {
-    aMetrics.mMaxElementWidth = aMetrics.width;
+    aMetrics.SetMEWToActualWidth(aReflowState.mStylePosition->mWidth.GetUnit());
   }
 }