Show AlertDialog when WebAPKs are enabled in chrome://flags but Unknown sources is disabled

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 2692 matching lines @@
       </message>
 
       <!-- Item Chooser UI strings -->
       <message name="IDS_ITEM_CHOOSER_ITEM_NAME_WITH_ID" desc="To distinguish items with the same name, the item chooser shows the item name with id.">
         <ph name="ITEM_NAME">%1$s<ex>item_name</ex></ph> (<ph name="ITEM_ID">%2$s<ex>item id</ex></ph>)
       </message>
 
       <message name="IDS_INSTANT_APPS_OPEN_IN_APP" desc="Button label for the user action that will open an Instant App.">
         Open app
       </message>
+
+      <!-- WebAPK related strings -->
+      <message name="IDS_WEBAPK_UNKNOWN_SOURCES_DIALOG_TITLE" desc="Title of dialog warning user that installation from Unknown sources is required for WebAPK experiment.">
+        Improved Add to Home screen Experiment
+      </message>
+
+      <message name="IDS_WEBAPK_UNKNOWN_SOURCES_DIALOG_MESSAGE" desc="Text of dialog warning user that installation from Unknown sources is required for WebAPK experiment">
+        Turn on “Unknown sources” in Android security settings to try "Improved Add to Home screen" experiment. Be sure to disable this setting when you’re done trying it out.
+      </message>
+
+      <message name="IDS_WEBAPK_UNKNOWN_SOURCES_SETTINGS_BUTTON" desc="Button label for opening Android Settings from dialog warning user that installation from Unknown sources is required for WebAPK experiment.">
+        Settings
+      </message>
     </messages>
   </release>
 </grit>