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

chrome/android/java/strings/android_chrome_strings.grd

Index: chrome/android/java/strings/android_chrome_strings.grd
diff --git a/chrome/android/java/strings/android_chrome_strings.grd b/chrome/android/java/strings/android_chrome_strings.grd
index b2b4fd38b41769c1acb55e5f4acac57cfc400a18..20a0c28c4cd386bea43806c62cb16f77b6dc8e2c 100644
--- a/chrome/android/java/strings/android_chrome_strings.grd
+++ b/chrome/android/java/strings/android_chrome_strings.grd
@@ -2710,6 +2710,19 @@ You can control the Physical Web in Chrome Settings.
       <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>