Fix undesired normalization of internal structure

It's undesired because it is used in other places to check if a comment
is in the same place.

Change-Id: Ic674acd307dedc1573aae61decaefec2f2b4234b

polygerrit-ui/app/elements/diff/gr-ranged-comment-layer/gr-ranged-comment-layer.js

@@ -167,6 +167,9 @@
       const ranges = this.get(['_rangesMap', side, lineNum]) || [];
       return ranges
           .map(range => {
+            // Make a copy, so that the normalization below does not mess with
+            // our map.
+            range = Object.assign({}, range);
             range.end = range.end === -1 ? line.text.length : range.end;
 
             // Normalize invalid ranges where the start is after the end but the