Switch to gr-icon from iron-icon

Gerrit 3.8 has largely deprecated iron-icons except for a small number
of cases which don't include the icons we are using in this plugin.
Switch to the material symbols font family provied through gr-icon which
appears to be the proper way to use check and close icons in Gerrit now.

This fixes missing icons under Gerrit 3.8. Gerrit 3.7 still renders them
using the old iron-icons but is expected to be compatibile with gr-icon
as well.

Change-Id: I96ffec351ac45293717b54fef73ef97bff80659c

web/plugin.js

diff --git a/web/plugin.js b/web/plugin.js
index 498a9ed..3a9907f 100644
--- a/web/plugin.js
+++ b/web/plugin.js
@@ -143,8 +143,8 @@
     <thead>
      <tr>
       <th>
-       <template is="dom-if" if="{{item.succeeded}}"><span style="color:green"><iron-icon icon="gr-icons:check"></iron-icon></span></template>
-       <template is="dom-if" if="{{!item.succeeded}}"><span style="color:red"><iron-icon icon="gr-icons:close"></iron-icon></span></template>
+       <template is="dom-if" if="{{item.succeeded}}"><gr-icon icon="check" style="color:var(--success-foreground)"></gr-icon></template>
+       <template is="dom-if" if="{{!item.succeeded}}"><gr-icon icon="close" style="color:var(--error-foreground)"></gr-icon></template>
        <b>[[item.author_name]]</b> on Patchset <b>[[item.revision]]</b> in pipeline <b>[[item.pipeline]]</b></th>
       <th><template is="dom-if" if="{{item.rechecks}}">[[item.rechecks]] rechecks</template></th>
       <th><span class="date"><gr-date-formatter show-date-and-time="" date-str="[[item.gr_date]]"></gr-date-formatter></span></th>