Replace gr-diff-comment-thread-group with div

This element is now nothing but an empty container, so I think it's
better to just use a div.

Change-Id: I58da47854d2c50608f9e217e7a12e30aad9aca98
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index b258455..80ca01d 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -36,6 +36,11 @@
       :host(.no-left) .sideBySide ::content .right:not([data-value]) + td {
         display: none;
       }
+      ::slotted(*) .thread-group {
+        display: block;
+        max-width: var(--content-width, 80ch);
+        white-space: normal;
+      }
       .diffContainer {
         display: flex;
         font-family: var(--monospace-font-family);
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index 557a652..76a62b8 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -365,7 +365,7 @@
     },
 
     _getThreadGroupForLine(contentEl) {
-      return contentEl.querySelector('gr-diff-comment-thread-group');
+      return contentEl.querySelector('.thread-group');
     },
 
     /**
@@ -378,7 +378,8 @@
       // Check if thread group exists.
       let threadGroupEl = this._getThreadGroupForLine(contentEl);
       if (!threadGroupEl) {
-        threadGroupEl = document.createElement('gr-diff-comment-thread-group');
+        threadGroupEl = document.createElement('div');
+        threadGroupEl.className = 'thread-group';
         contentEl.appendChild(threadGroupEl);
       }
       return threadGroupEl;
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.html
index 316990d..4befd2f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.html
@@ -315,8 +315,7 @@
         // The new thread group can be fetched.
         assert.isOk(element._getThreadGroupForLine(contentEl));
 
-        assert.equal(contentEl.querySelectorAll(
-            'gr-diff-comment-thread-group').length, 1);
+        assert.equal(contentEl.querySelectorAll('.thread-group').length, 1);
       });
 
       suite('image diffs', () => {