Add string for VoiceOver announcement on page loaded.

ios/chrome/app/strings/ios_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
 This file contains definition of resources that will be translated for each
 locale.  The strings in this file are specific to iOS.
 -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       output_all_resource_defines="false" source_lang_id="en" enc_check="möl">
   <outputs>
@@ skipping 887 matching lines @@
       </message>
       <message name="IDS_IOS_OPTIONS_SEND_USAGE_DATA" desc="Title for opening the setting for if/when to send usage data and crash reports. [Length: 20em] [iOS only]">
         Send Usage Data
       </message>
       <message name="IDS_IOS_OPTIONS_WEB_SERVICES_LABEL" desc="Title for group of web services items in Privacy settings table, including Show Suggestions, Preload Webpages, and Send Usage Data. [Length: 10em] [iOS only]">
         Web Services
       </message>
       <message name="IDS_IOS_PAGE_INFO_SECURITY_BUTTON_ACCESSIBILITY_LABEL" desc="The accessibility text for the page security info button">
         Page Security Info
       </message>
+      <message name="IDS_IOS_PAGE_LOADED_ACCESSIBILITY_ANNOUNCEMENT" desc="The accessibility announcement when a page has loaded. [Length: unlimited] [iOS only]">
+        Webpage loaded
+      </message>
       <message name="IDS_IOS_PASSWORD_MANAGER_BLACKLIST_BUTTON" desc="Button text for the 'Save Password' infobar's 'Never remember for this site' option [Length: 10em]">
         Never
       </message>
       <message name="IDS_IOS_PASSWORD_MANAGER_SAVE_BUTTON" desc="Save button text for password manager [Length: 10em]">
         Save
       </message>
       <message name="IDS_IOS_PASSWORD_MANAGER_SAVE_PASSWORD_PROMPT_SMART_LOCK_BRANDING" desc="Info bar message to save a password, referring to the Google Smart Lock brand. [Length: 70em]">
         Do you want Google Smart Lock to save your password for this site?
       </message>
       <message name="IDS_IOS_PASSWORD_MANAGER_SMART_LOCK_FOR_PASSWORDS" desc="The more descriptive brand name of the password manager when the user is signed in. Used in various UI elements, can be in link format.">
@@ skipping 584 matching lines @@
       </message>
       <message name="IDS_IOS_VOICE_SEARCH_SETTING_TITLE" desc="The title for Voice Search setting [Length: 29em] [iOS only]">
         Voice Search
       </message>
       <message name="IDS_IOS_VOICE_SEARCH_SETTING_TTS" desc="The label for setting switch enabling text-to-speech response after voice search [Length: 29em] [iOS only]">
         Speak Answers Back
       </message>
     </messages>
   </release>
 </grit>