[Presentation API] (1st) (1-UA) Add is_offscreen_presentation attribute to MediaRouter

extensions/renderer/resources/media_router_bindings.js

Index: extensions/renderer/resources/media_router_bindings.js
diff --git a/extensions/renderer/resources/media_router_bindings.js b/extensions/renderer/resources/media_router_bindings.js
index 3dcc0f6a822dec23313b7e4c1e3f2acd6cb3c02a..825a9e0f5c398a7445c66fb5f724af08e29beebf 100644
--- a/extensions/renderer/resources/media_router_bindings.js
+++ b/extensions/renderer/resources/media_router_bindings.js
@@ -80,7 +80,8 @@ define('media_router_bindings', [
       // Begin newly added properties, followed by the milestone they were
       // added.  The guard should be safe to remove N+2 milestones later.
       'for_display': route.forDisplay, // M47
-      'is_incognito': !!route.offTheRecord  // M50
+      'is_incognito': !!route.offTheRecord,  // M50
+      'is_offscreen_presentation': !!route.isOffscreenPresentation // M56
     });
   }