Cancel comment updates on discard

Fixes race condition triggering re-rendering of a deleted comment by
canceling debounced updates for `comment.editing` changes.

Bug: Issue 4319
Change-Id: I2a7578443783360088d6f65decd7dcb8be5e636b
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 a2d7fd0..1b30bde 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
@@ -280,6 +280,7 @@
     },
 
     _fireDiscard: function() {
+      this.cancelDebouncer('fire-update');
       this.fire('comment-discard', this._getEventPayload());
     },