Bug 1385369 - Reserve 1 space in Selection::mRanges; r=qdot

Most selections on the Web end up with one Range inside them.
By reserving the space for this one range inline, we can avoid the
allocator pressure in a lot of hot code when manipulating the
Selection object.

dom/base/Selection.h

@@ -437,7 +437,7 @@ private:
   // proves to be a performance concern, then an interval tree may be a
   // possible solution, allowing the calculation of the overlap interval in
   // O(log n) time, though this would require rebalancing and other overhead.
-  nsTArray<RangeData> mRanges;
+  AutoTArray<RangeData, 1> mRanges;
 
   RefPtr<nsRange> mAnchorFocusRange;
   RefPtr<nsFrameSelection> mFrameSelection;