Implement virtual keyboard hiding.

ui/keyboard/resources/api_adapter.js

 // 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.
 
 function logIfError() {
   if (chrome.runtime.lastError) {
     console.log(chrome.runtime.lastError);
   }
 }
 
 function insertText(text) {
   chrome.virtualKeyboardPrivate.insertText(text, logIfError);
 }
 
 function MoveCursor(swipe_direction, swipe_flags) {
   chrome.virtualKeyboardPrivate.moveCursor(swipe_direction, swipe_flags);
 }
 
 function sendKeyEvent(event) {
   chrome.virtualKeyboardPrivate.sendKeyEvent(event, logIfError);
 }
+
+function hideKeyboard() {
+  chrome.virtualKeyboardPrivate.hideKeyboard(logIfError);
+}