Coalesce request for drafts with login check

Because requests for a user's draft diff comments is always nested
within a check for whether a user is logged in, these can be combined
into the same REST API interface call. The updated call is given a JSDoc
to describe the additional functionality and call sites are simplified.

Change-Id: Idc0cc6298c7287579ba76e7cc35701e62142bca3
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 e4901f5..8655aa0 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
@@ -730,10 +730,7 @@
     },
 
     _getDiffDrafts() {
-      return this._getLoggedIn().then(loggedIn => {
-        if (!loggedIn) { return Promise.resolve({}); }
-        return this.$.restAPI.getDiffDrafts(this._changeNum);
-      });
+      return this.$.restAPI.getDiffDrafts(this._changeNum);
     },
 
     _computeCommentSkips(commentMap, fileList, path) {