Bug 1274397 - Small optimization to use a RegionBuilder. r=tnikkel

MozReview-Commit-ID: 6Yro9uDIIEd

layout/base/nsLayoutUtils.cpp

@@ -113,6 +113,7 @@
 #include "mozilla/RuleNodeCacheConditions.h"
 #include "mozilla/StyleSetHandle.h"
 #include "mozilla/StyleSetHandleInlines.h"
+#include "RegionBuilder.h"
 
 #ifdef MOZ_XUL
 #include "nsXULPopupManager.h"
@@ -9064,14 +9065,14 @@ nsLayoutUtils::TransformToAncestorAndCombineRegions(
     return;
   }
   bool isPrecise;
-  nsRegion transformedRegion;
+  RegionBuilder<nsRegion> transformedRegion;
   for (nsRegion::RectIterator it = aRegion.RectIter(); !it.Done(); it.Next()) {
     nsRect transformed = TransformFrameRectToAncestor(
       aFrame, it.Get(), aAncestorFrame, &isPrecise, aMatrixCache);
     transformedRegion.OrWith(transformed);
   }
   nsRegion* dest = isPrecise ? aPreciseTargetDest : aImpreciseTargetDest;
-  dest->OrWith(transformedRegion);
+  dest->OrWith(transformedRegion.ToRegion());
 }
 
 /* static */ bool