Add reloadDrafts function in gr-comment-api

This function also re-initializes a ChangeComment object, but uses the
old values for comments and robot comments, and only makes a request for
new drafts.

It's response is identical to loadAll for the purposes of elements using
these functions, but will reduce unneeded API calls.

Change-Id: I3c0872b86a0e24c9c378acf3311fe8cc3e078eda

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

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 7c46b58..5431a78 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
@@ -751,8 +751,8 @@
     },
 
     _loadComments() {
-      return this.$.commentAPI.loadAll(this._changeNum).then(() => {
-        this._changeComments = this.$.commentAPI._changeComments;
+      return this.$.commentAPI.loadAll(this._changeNum).then(comments => {
+        this._changeComments = comments;
         this._commentMap = this._getPaths(this._patchRange);
 
         this._commentsForDiff = this._getCommentsForPath(this._path,