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 ee5b1aeb2a0991a197218561336ed0ba50ff9e06..5e45847979594289ec463716f4c12379926c2a34 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -4868,6 +4868,12 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_AURA_WINDOWS_DESCRIPTION" desc="Description for the flag to enable fancy translucent windows.">
         Enable the "Aura" style for translucent window frames and constrained window dragging.
       </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.">