Fixes modifier handling for repeat keys.

ui/keyboard/resources/elements/kb-keyset.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-keyset" attributes="nextKeyset isDefault"
     on-key-up="keyUp" on-key-longpress="keyLongpress">
   <template>
     <style>
       @host {
         * {
           -webkit-box-orient: vertical;
           -webkit-box-flex: 1;
           display: -webkit-box;
         }
       }
     </style>
     <content select="kb-row"></content>
     <content select="kb-altkey-container" id="altkeyContainer"
         touch-action="none"></content>
     <kb-altkey-data id="altkeyMetadata"></kb-altkey-data> 
   </template>
   <script>
     Polymer('kb-keyset', {
       isDefault: false,
       nextKeyset: undefined,
       // TODO(bshe): support select keyset on down, long and dbl events.
       keyUp: function(event, detail) {
+        switch (detail.char) {
+          case 'Shift':
+          case 'Alt':
+          case 'Ctrl':
+            return;
+          default:
+            break;
+        }
         if (!detail.toKeyset)
           detail.toKeyset = this.nextKeyset;
       },
       keyLongpress: function(event, detail) {
         if (!detail.char)
           return;
 
         var altkeyContainer = this.$.altkeyContainer.getDistributedNodes()[0];
         if (!altkeyContainer)
           return;
@@ skipping 28 matching lines @@
         var leftOffset = activeAltKeySet.offset * event.target.clientWidth;
         activeAltKeySet.style.left = event.target.offsetLeft - leftOffset +
             'px';
         var nodes = activeAltKeySet.childNodes;
         nodes[activeAltKeySet.offset].classList.add('active');
         altkeyContainer.hidden = false;
       }
     });
   </script>
 </polymer-element>