Explicitly make file comments dependent on the comments prop

Not doing so was causing weird behavior where comments from other
authors were showing up in the comments list.

Also clean up some small nits around the gr-comment-list code.

Change-Id: Ida428b347e75b705dcecef0d6a52936973aa068c
diff --git a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
index 263fb28..da15406 100644
--- a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
+++ b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
@@ -42,12 +42,12 @@
         word-wrap: break-word;
       }
     </style>
-    <template is="dom-repeat" items="{{_files}}" as="file">
+    <template is="dom-repeat" items="[[_files]]" as="file">
       <div class="file">
         <a href$="[[_computeFileDiffURL(file, changeNum, patchNum)]]">[[file]]</a>:
       </div>
       <template is="dom-repeat"
-                items="[[_computeCommentsForFile(file)]]" as="comment">
+                items="[[_computeCommentsForFile(comments, file)]]" as="comment">
         <div class="container">
           <a class="lineNum"
              href$="[[_computeDiffLineURL(file, changeNum, comment.patch_set, comment)]]">
diff --git a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
index b40c18e..c23c373 100644
--- a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
+++ b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
@@ -19,17 +19,17 @@
 
     properties: {
       changeNum: Number,
-      comments: {
-        type: Object,
-        observer: '_commentsChanged',
-      },
+      comments: Object,
       patchNum: Number,
 
-      _files: Array,
+      _files: {
+        type: Array,
+        computed: '_computeFiles(comments)',
+      },
     },
 
-    _commentsChanged: function(value) {
-      this._files = Object.keys(value || {}).sort();
+    _computeFiles: function(comments) {
+      return Object.keys(comments || {}).sort();
     },
 
     _computeFileDiffURL: function(file, changeNum, patchNum) {
@@ -45,8 +45,8 @@
       return diffURL;
     },
 
-    _computeCommentsForFile: function(file) {
-      return this.comments[file];
+    _computeCommentsForFile: function(comments, file) {
+      return comments[file];
     },
 
     _computePatchDisplayName: function(comment) {