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 2339c426250c897f9cb5c227c8dc4195294c113d..2760ce848261edf5dfc45b2ca89067bdab425768 100644
--- a/chrome/android/java/strings/android_chrome_strings.grd
+++ b/chrome/android/java/strings/android_chrome_strings.grd
@@ -2706,6 +2706,21 @@ 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_MESSAGE" desc="Text of dialog warning user that installation from Unknown sources is required for WebAPK experiment.">
+        "Installation from Unknown Sources" is required in order to install WebAPKs from the "Add to Homescreen" app menu item.
+
+        Enabling "Installation from Untrusted Sources" makes your device more vulnerable. Be sure to re-enable installation from unknown sources once you are done playing with the "WebAPK experiment".

[gone, 2016/11/04 22:07:43]

1) You don't want users to "re-enable installation from unknown sources"; they
had to turn it on in order to play with WebAPKs.

2) Untrusted or Unknown?  You flip between the two repeatedly in your CL, and
even in this message.  Pick one.

+      </message>
+
+      <message name="IDS_WEBAPK_UNKNOWN_SOURCES_DIALOG_TITLE" desc="Title of dialog warning user that installation from Unknown sources is required for WebAPK experiment.">
+        WebAPK Experiment
+      </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>