Hook up topic removal to backend

Change-Id: I93d8e7ef09bd204cf5b5c022cb3437f50949c5bd
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
index e458068..b56324a 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
@@ -180,6 +180,7 @@
 
     _handleTopicRemoved: function() {
       this.set(['change', 'topic'], '');
+      this.$.restAPI.setChangeTopic(this.change.change_id, null);
     },
   });
 })();
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_test.html b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_test.html
index 04cc212..d354fd7 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_test.html
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata_test.html
@@ -175,11 +175,12 @@
       });
 
       test('clicking x on topic chip removes topic', function() {
-        sandbox.stub(element, '_handleTopicChanged');
+        var topicStub = sandbox.stub(element.$.restAPI, 'setChangeTopic');
         flushAsynchronousOperations();
         var remove = element.$$('gr-linked-chip').$.remove;
         MockInteractions.tap(remove);
         assert.equal(element.change.topic, '');
+        assert.isTrue(topicStub.called);
       });
     });
   });