Disallow tabbing on paper-button

Because you can already tab through gr-buttons, the paper-button made it
so that each button had two tabs. The tabindex should only be on the
gr-buttons, so disallow on paper-button. For easy demonstration of this
issue, try to tab through the labels on the reply dilog.

Change-Id: Ibc1e7d753f3cda1786d09f81433656c9d3282305
(cherry picked from commit 048e8dc960e1c7df14977fbeaa92d9b983a86236)

polygerrit-ui/app/elements/shared/gr-button/gr-button.html

diff --git a/polygerrit-ui/app/elements/shared/gr-button/gr-button.html b/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
index c0b17af..9be6497 100644
--- a/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
+++ b/polygerrit-ui/app/elements/shared/gr-button/gr-button.html
@@ -116,7 +116,7 @@
         color: #aaa;
       }
     </style>
-    <paper-button raised="[[!link]]" disabled="[[disabled]]">
+    <paper-button raised="[[!link]]" disabled="[[disabled]]" tabindex="-1">
       <content></content>
       <i class="downArrow"></i>
     </paper-button>