Save top-level draft change comments in localstorage

Unsaved diff comment drafts are automatically backed-up in local-storage
if the user discards them on accident or navigates away without saving
them. This change extends the same functionality to change-level
comments and adds tests accordingly.

Bug: Issue 4258
Change-Id: Iecfdecfc10cd92f798f1f7306af994b6ec8f74db

polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js

diff --git a/polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js b/polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js
index a2afb89..ff41a74 100644
--- a/polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js
+++ b/polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js
@@ -58,7 +58,7 @@
 
     _getDraftKey: function(location) {
       return ['draft', location.changeNum, location.patchNum, location.path,
-          location.line].join(':');
+          location.line || ''].join(':');
     },
 
     _cleanupDrafts: function() {