Add shadowRootReady callback for SkyElement.

sky/tests/resources/test-element.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 
 <sky-element name="test-element">
 <template>
   <div id="inside" on-test-event="handleEvent" attr="{{ value }}">{{ value }}</div>
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     this.lastEvent = null;
     this.value = 10;
+    this.shadowRootReadyCount = 0;
   }
   handleEvent(event) {
     this.lastEvent = event;
   }
+  shadowRootReady() {
+    this.shadowRootReadyCount++;
+  }
 }.register();
 </script>
 </sky-element>