Switch Sky to pointer events

sky/framework/sky-checkbox/sky-checkbox.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-button/sky-button.sky" as="SkyButton" />
 
 <sky-element
     name="sky-checkbox"
     attributes="checked:boolean"
-    on-mouseup="handleMouseUp">
+    on-click="handleClick">
 <template>
   <style>
     :host {
       display: flex;
       justify-content: center;
       align-items: center;
       -webkit-user-select: none;
       cursor: pointer;
       width: 30px;
       height: 30px;
@@ skipping 36 matching lines @@
 
     this.containerStyle = '';
     this.checkStyle = '';
   }
   checkedChanged(oldValue, newValue) {
     this.checkStyle = newValue ? 'checked' : '';
   }
   highlightChanged(oldValue, newValue) {
     this.containerStyle = newValue ? 'highlight' : '';
   }
-  handleMouseUp() {
+  handleClick() {
     this.checked = !this.checked;
   }
 }.register();
 </script>
 </sky-element>