Add platform_app flag to manifest.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 9218c9d3209b41ab37071e13a233159e94602fa7..fc4bc8dd34e102de713ca1fcc1e532937edae5a9 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -4838,6 +4838,12 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_ENABLE_MEDIA_SOURCE_DESCRIPTION" desc="Description for the flag to enable the Media Source API on &lt;video&gt; elements.">
         Enable experimental Media Source API on the video elements. This API allows JavaScript to send media data directly to a video element.
       </message>
+      <message name="IDS_FLAGS_ENABLE_PLATFORM_APPS_NAME" desc="Title for the flag to enable platform apps.">
+        Enable platform apps
+      </message>
+      <message name="IDS_FLAGS_ENABLE_PLATFORM_APPS_DESCRIPTION" desc="Description for the flag to enable platform apps.">
+        Enable advanced application capabilities.
+      </message>
 
       <!-- Crashes -->
       <message name="IDS_CRASHES_TITLE" desc="Title for the chrome://crashes page.">