Refactor CSS rules for the shift-key.

ui/keyboard/resources/elements/kb-key.html

 <!--
   -- Copyright 2013 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.
   -->
 
 <polymer-element name="kb-key" extends="kb-key-base" attributes="image keyCode
     keyName shiftModifier sound stretch weight">
   <template>
     <style>
@@ skipping 11 matching lines @@
         background-image: none;
         background-position: center;
         background-repeat: no-repeat;
         background-size: contain;
         bottom: 0;
         height: 1.2em;
         left: 0;
         margin: auto;
         position: absolute;
         right: 0;
+        text-align: center;
         top: 0;
       }
 
       :host([align=left]) .key {
         padding-left: 1em;
       }
 
       /* TODO(rsadam@): Remove when we fully switch to native shadow dom. */
       /* TODO(rsadam@): Move shift key rules to kb-shift-key. */
       kb-shift-key .key,
@@ skipping 66 matching lines @@
 </polymer-element>
 
 <!-- Special keys -->
 <polymer-element name="kb-abc-key" class="symbol dark" char="Invalid"
     extends="kb-key" weight="125">
 </polymer-element>
 
 <polymer-element name="kb-hide-keyboard-key" class="hide-keyboard dark"
     align="center" attributes="showMenu" char="Invalid" extends="kb-key">
 </polymer-element>