[Custom Tabs] Implement "Read It Later" Behind a Flag

chrome/android/java/strings/android_chrome_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- android_chrome_strings.grd contains strings for Chrome for Android.
 
 FAQ:
   - How do I add, change, or remove a string?
       Just update this file.
   - Translations will happen automatically??
       Yes, indeed. Just add your string before the branch point and
       make the description good.
@@ skipping 2254 matching lines @@
       </message>
       <message name="IDS_NEW_TAB_INCOGNITO_HEADER" desc="Header shown when a user opens an incognito tab explaining incognito mode">
         You’ve gone incognito.
       </message>
       <message name="IDS_NEW_TAB_INCOGNITO_MESSAGE" desc="Message shown when a user opens an incognito tab explaining incognito mode">
         Pages you view in incognito tabs won’t stick around in your browser’s history, cookie store, or search history after you’ve closed all of your incognito tabs. Any files you download or bookmarks you create will be kept.
 
 However, you aren’t invisible. Going incognito doesn’t hide your browsing from your employer, your internet service provider, or the websites you visit.
       </message>
 
+      <!-- Custom tabs -->
+      <message name="IDS_READ_IT_LATER" desc="Text describing an option for the user to save the tab to Chrome and read it in the future.">
+        Read it later in Chrome
+      </message>
+      <message name="IDS_TAB_SENT_TO_BACKGROUND" desc="A message to show that the tab has been sent to the background Chrome.">
+        Your tab is now in the background.
+      </message>
+
       <!-- Contextual Search -->
       <message name="IDS_CONTEXTUAL_SEARCH_NETWORK_UNAVAILABLE" desc="Tells the user the network is not accessible.">
         Unable to access the network
       </message>
       <message name="IDS_CONTEXTUAL_SEARCH_ERROR" desc="A network error message">
         Error: <ph name="ERROR_CODE">%1$s<ex>404</ex></ph>
       </message>
 
       <!-- Web apps -->
       <message name="IDS_WEBAPP_ACTIVITY_TITLE" desc="Title in recent tasks list for web apps, i.e. web pages that are shown in a separate window">
@@ skipping 238 matching lines @@
       <!-- Migration strings -->
       <message name="IDS_TAB_SWITCHER_CALLOUT_HEADER" desc="Header for the Tab Switcher callout.">
         Quickly switch tabs
       </message>
       <message name="IDS_TAB_SWITCHER_CALLOUT_BODY" desc="Indicates that clicking the tab switcher button gives you quick access to your tabs.">
         Tap this button for quick access to your tabs.
       </message>
     </messages>
   </release>
 </grit>