Add paper-tabs dependency

This will be used for tabbing between the new comment thread view and
the current message list.

Change-Id: Ib18024a747bc9f647f5588651e4959dcdcae8b5c

polygerrit-ui/BUILD

diff --git a/polygerrit-ui/BUILD b/polygerrit-ui/BUILD
index 0c2cd5e..7487ad5 100644
--- a/polygerrit-ui/BUILD
+++ b/polygerrit-ui/BUILD
@@ -26,6 +26,7 @@
         "//lib/js:paper-input",
         "//lib/js:paper-item",
         "//lib/js:paper-listbox",
+        "//lib/js:paper-tabs",
         "//lib/js:paper-toggle-button",
         "//lib/js:polymer",
         "//lib/js:polymer-resin",