Fix missing import in path-list-behavior

This change fixes a bug that was causing tests to fail in
gr-path-list-behavior in Safari. Because html imports are polyfilled
by Polymer, webcomponents.js must be included in tests -- even though no
other Polymer-specific functionality is needed.

Change-Id: Ic21aca54114aa753b3ca446a845c1a288b96894a
diff --git a/polygerrit-ui/app/behaviors/gr-path-list-behavior/gr-path-list-behavior_test.html b/polygerrit-ui/app/behaviors/gr-path-list-behavior/gr-path-list-behavior_test.html
index 530b7be..adf0bf1 100644
--- a/polygerrit-ui/app/behaviors/gr-path-list-behavior/gr-path-list-behavior_test.html
+++ b/polygerrit-ui/app/behaviors/gr-path-list-behavior/gr-path-list-behavior_test.html
@@ -13,6 +13,8 @@
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
+<!-- Polymer included for the html import polyfill. -->
+<script src="../../../bower_components/webcomponentsjs/webcomponents.min.js"></script>
 <script src="../../bower_components/web-component-tester/browser.js"></script>
 <title>gr-path-list-behavior</title>