Remove draft comment from local storage upon discard

Previously, when a user discarded a draft comment, the draft was still
stored in local storage. If they tried to reply to a comment on the same
line, the previous message appeared when it shouldn't have. This change
removes drafts from local storage when they are discarded.

Bug: Issue 4361
Change-Id: Id234676972a21a879e68e754968abf7008098cf6
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
index 8905854..72138ff 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.js
@@ -125,12 +125,7 @@
       this.comment.message = this._messageText;
       this.disabled = true;
 
-      this.$.storage.eraseDraftComment({
-        changeNum: this.changeNum,
-        patchNum: this.patchNum,
-        path: this.comment.path,
-        line: this.comment.line,
-      });
+      this._eraseDraftComment();
 
       this._xhrPromise = this._saveDraft(this.comment).then(function(response) {
         this.disabled = false;
@@ -155,6 +150,15 @@
       }.bind(this));
     },
 
+    _eraseDraftComment: function() {
+      this.$.storage.eraseDraftComment({
+        changeNum: this.changeNum,
+        patchNum: this.patchNum,
+        path: this.comment.path,
+        line: this.comment.line,
+      });
+    },
+
     _commentChanged: function(comment) {
       this.editing = !!comment.__editing;
       if (this.editing) { // It's a new draft/reply, notify.
@@ -338,6 +342,8 @@
       }
       this.editing = false;
       this.disabled = true;
+      this._eraseDraftComment();
+
       if (!this.comment.id) {
         this.disabled = false;
         this._fireDiscard();