Add 2px rounded border to the "draft" comment bubble

Now that the box is larger than the text "draft" marker it looks
fine to use a small rounded border on the corners of the text that
says "draft", making it more bubble like in appearance.

Change-Id: I9f8e55ec1c6283c84ff9ad6e40306bcebd60bc41

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DraftBox.ui.xml

@@ -26,6 +26,7 @@ limitations under the License.
       text-align: center;
       color: #fff;
       background-color: #aaa;
+      -webkit-border-radius: 2px;
     }
     .editArea { max-width: 637px; }
     button.button div {