ChangeScreen2: Fix missing ellipsis on long subject lines

white-space must be set to nowrap to obtain an ellipsis when overflow
is hidden. Add the missing property to the subject CSS class.

Change-Id: I50e293440748986eaebbbf0a15dbee634c2b9dc8

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen2.ui.xml

@@ -51,6 +51,7 @@ limitations under the License.
       line-height: HEADER_HEIGHT;
       overflow: hidden;
       text-overflow: ellipsis;
+      white-space: nowrap;
     }
     .subjectButtons {
       position: absolute;