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..cf5584378634a0a10ac7cf204a421dfa87b49f98 100644
--- a/chrome/android/java/strings/android_chrome_strings.grd
+++ b/chrome/android/java/strings/android_chrome_strings.grd
@@ -2706,6 +2706,23 @@ 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_CANARY" desc="Text of dialog warning user that installation from Unknown sources is required for WebAPK experiment on Chrome Canary.">
+        Turn on “Unknown sources” in Android security settings to try "Improved Add to Home screen" experiment in Chrome Canary. Be sure to disable this setting when you’re done trying it out.
+      </message>
+
+      <message name="IDS_WEBAPK_UNKNOWN_SOURCES_DIALOG_MESSAGE_DEV" desc="Text of dialog warning user that installation from Unknown sources is required for WebAPK experiment on Chrome Dev.">
+        Turn on “Unknown sources” in Android security settings to try "Improved Add to Home screen" experiment in Chrome Dev. Be sure to disable this setting when you’re done trying it out.

[gone, 2016/11/07 18:27:13]

Not much value in differentiating between Canary in Dev in these strings.  Just
make it one string again.

[pkotwicz, 2016/11/07 20:08:13]

What are you suggesting changing the string to?

[gone, 2016/11/07 20:59:29]

Take out the "Dev"/"Canary" bits, which will collapse both into one string.

[gone, 2016/11/07 21:12:50]

I meant to keep Chrome, but to remove the channel.  lgtming this anyway because
the string still works.

+      </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>