Add iOS strings for contextual search in chrome://flags.

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 variables is_win, is_macosx, is_linux, is_ios and is_posix are
 available for making strings OS specific.  Other platform defines such as
 use_titlecase are declared in build/common.gypi.
 
 The strings in this file are specific to iOS, thus is_ios will be true and
 all other OS specific variables will be false.
@@ skipping 157 matching lines @@
       <message name="IDS_IOS_KEYBOARD_OPEN_LOCATION" desc="Title of the keyboard shortcut to open a location (puts cursor in the URL bar). [Length: 20 em] [iOS only]">
         Open Location...
       </message>
       <message name="IDS_IOS_KEYBOARD_REOPEN_CLOSED_TAB" desc="Title of the keyboard shortcut to reopen a recently closed tab. [Length: 20 em] [iOS only]">
         Reopen Closed Tab
       </message>
       <message name="IDS_IOS_LOCATION_USAGE_CANCEL" desc="Title for the alert button to cancel sending location for Omnibox queries [Length: 20em] [iOS only]">
         Not Now
       </message>
 
+      <message name="IDS_IOS_FLAGS_CONTEXTUAL_SEARCH" desc="Title for the flag to enable Contextual Search.">
+        Contextual Search
+      </message>
+      <message name="IDS_IOS_FLAGS_CONTEXTUAL_SEARCH_DESCRIPTION" desc="Description for the flag to enable Contextual Search.">
+        Whether or not Contextual Search is enabled.
+      </message>
+
       <!-- iOS Net-Export strings -->
       <message name="IDS_IOS_NET_EXPORT_NO_EMAIL_ACCOUNTS_ALERT_MESSAGE" desc="The message of the alert informing a user with no email accounts that he or she needs to configure an email account to send net-export data. [Length: 70em]">
         To send net-export data, please configure your email account in the Settings app.
       </message>
       <message name="IDS_IOS_NET_EXPORT_NO_EMAIL_ACCOUNTS_ALERT_TITLE" desc="The title of the alert informing a user with no email accounts that he or she needs to configure an email account to send net-export data. [Length: 20em]">
         No Email Accounts
       </message>
 
       <message name="IDS_IOS_VERSION_COPIED" desc="The message displayed when the version label is tapped (in About settings) [Length: 10em] [iOS only]">
         Copied
@@ skipping 19 matching lines @@
       </message>
       <message name="IDS_IOS_GENERATED_PASSWORD_PROMPT_DESCRIPTION" desc="The explanation text for the generated password prompt. [iOS only]">
         Chrome will store this in your <ph name="BEGIN_LINK">BEGIN_LINK</ph>Google saved passwords<ph name="END_LINK">END_LINK</ph> and autofill it for you so you don't have to remember it yourself.
       </message>
       <message name="IDS_IOS_GENERATED_PASSWORD_PROMPT_TITLE" desc="The text of the button that accepts the generated password. [Length: 20em] [iOS only]">
         Password Generated by Chrome
       </message>
     </messages>
   </release>
 </grit>