Bug 1432341 - Simplify by using GetContentRectRelativeToSelf. r=mattwoodrow

This is just cleanup in advance of the patch to fix this bug.

MozReview-Commit-ID: 1pzauGix51m

--HG--
extra : rebase_source : b36a4ef7da6fb6b6ef7a1a82c56aa489e10935d5

layout/generic/nsColumnSetFrame.cpp

@@ -167,9 +167,7 @@ nsColumnSetFrame::ForEachColumn(const std::function<void(const nsRect& lineRect)
   bool isVertical = wm.IsVertical();
   bool isRTL = !wm.IsBidiLTR();
 
-  // Get our content rect as an absolute coordinate, not relative to
-  // our parent (which is what the X and Y normally is)
-  nsRect contentRect = GetContentRect() - GetRect().TopLeft() + aPt;
+  nsRect contentRect = GetContentRectRelativeToSelf() + aPt;
   nsSize ruleSize = isVertical ? nsSize(contentRect.width, ruleWidth)
                                : nsSize(ruleWidth, contentRect.height);