Make sure threads have commentSide attribute

This should be set on the thread level because in a unified view, it is
possible for a thread group to contain threads from both the 'left' and
the 'right' sides.

Change-Id: Ia1f7d54423f5f424c98134224666d7f76c56de05

polygerrit-ui/app/elements/diff/gr-diff-comment-thread-group/gr-diff-comment-thread-group.html

@@ -32,7 +32,7 @@ limitations under the License.
     <template is="dom-repeat" items="[[_threads]]" as="thread">
       <gr-diff-comment-thread
           comments="[[thread.comments]]"
-          comment-side="[[commentSide]]"
+          comment-side="[[thread.commentSide]]"
           is-on-parent="[[isOnParent]]"
           parent-index="[[parentIndex]]"
           change-num="[[changeNum]]"

polygerrit-ui/app/elements/diff/gr-diff-comment-thread-group/gr-diff-comment-thread-group.js

@@ -19,7 +19,6 @@
 
     properties: {
       changeNum: String,
-      commentSide: String,
       comments: {
         type: Array,
         value() { return []; },
@@ -51,9 +50,10 @@
      *
      * @param {!Object} opt_range
      */
-    addNewThread(opt_range) {
+    addNewThread(commentSide, opt_range) {
       this.push('_threads', {
         comments: [],
+        commentSide,
         patchNum: this.patchForNewThreads,
         range: opt_range,
       });

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

@@ -448,7 +448,7 @@
       let threadEl = this._getThread(threadGroupEl, opt_range);
 
       if (!threadEl) {
-        threadGroupEl.addNewThread(opt_range);
+        threadGroupEl.addNewThread(commentSide, opt_range);
         Polymer.dom.flush();
         threadEl = this._getThread(threadGroupEl, opt_range);
       }