Add a default share button to the menu of Custom Tabs

chrome/android/java/res/menu/custom_tabs_menu.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 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. -->
 
 <menu xmlns:android="http://schemas.android.com/apk/res/android">
         <item android:id="@+id/icon_row_menu_id"
             android:title="@null"
             android:orderInCategory="0" >
             <menu>
               <item android:id="@+id/forward_menu_id"
                 android:title="@string/accessibility_menu_forward"
                 android:icon="@drawable/btn_forward"/>
               <item android:id="@+id/info_menu_id"
                 android:title="@string/accessibility_menu_info"
                 android:icon="@drawable/btn_info" />
               <item android:id="@+id/reload_menu_id"
                 android:title="@string/accessibility_btn_refresh"
                 android:icon="@drawable/btn_reload_stop"/>
             </menu>
         </item>
+        <item android:id="@+id/share_menu_id"
+            android:title="@string/menu_share_page"
+            android:orderInCategory="2" />
         <item android:id="@+id/find_in_page_id"
             android:title="@string/menu_find_in_page"
             android:orderInCategory="2" />
         <!-- Title is intentionally left blank in xml and will be set in java. -->
         <item android:id="@+id/open_in_chrome_id"
             android:title=""
             android:orderInCategory="2" />
 </menu>