Ignore triple clicks for other gr-diff-highlight instances

At the moment, gr-diff-highlight react to selection change event, so if
there are multiple instances on the page, for example in gr-change-view,
all instances react to selection.
This change makes gr-diff-highlight to ignore selections for other
instances, thus reacting only its own selection.

Bug: Issue 5504
Change-Id: I9f0e1dc7fe9f316400d9c96fb0c8c5f7ff78d779

polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.js

@@ -116,6 +116,10 @@
      * @return {!Object} fixed normalized range
      */
     _fixTripleClickSelection: function(range, domRange) {
+      if (!range.start) {
+        // Selection outside of current diff.
+        return range;
+      }
       var start = range.start;
       var end = range.end;
       var endsAtOtherSideLineNum =