Switch Sky to pointer events

sky/framework/sky-button/sky-button.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="sky-button"
     attributes="highlight:boolean"
-    on-mousedown="handleMouseDown"
-    on-mouseup="handleMouseUp"
-    on-mouseout="handleMouseOut">
+    on-pointerdown="handlePointerDown"
+    on-pointerup="handlePointerUp"
+    on-pointercancel="handlePointerCancel">
 <template>
   <style>
     :host {
       display: inline-flex;
       border-radius: 4px;
       justify-content: center;
       align-items: center;
       border: 1px solid blue;
       -webkit-user-select: none;
       margin: 5px;
     }
     :host([highlight=true]) {
       background-color: orange;
     }
   </style>
   <content />
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     super.created();
 
     this.tabIndex = 0; // Make focusable.
   }
-  handleMouseDown() {
+  handlePointerDown() {
     this.highlight = true;
   }
-  handleMouseUp() {
+  handlePointerUp() {
     this.highlight = false;
   }
-  handleMouseOut() {
+  handlePointerCancel() {
     this.highlight = false;
   }
 }.register();
 </script>
 </sky-element>