Fix hangs in Mandoline page cycler on Linux with --enable-multiprocess.

mojo/application/public/interfaces/content_handler.mojom

Index: mojo/application/public/interfaces/content_handler.mojom
diff --git a/mojo/application/public/interfaces/content_handler.mojom b/mojo/application/public/interfaces/content_handler.mojom
index 4281bf6278e39589b84a4ced5fd3fcdbf30939dc..7494a7bf027976133acce92e3adc4dd6675f11cd 100644
--- a/mojo/application/public/interfaces/content_handler.mojom
+++ b/mojo/application/public/interfaces/content_handler.mojom
@@ -7,6 +7,14 @@ module mojo;
 import "mojo/application/public/interfaces/application.mojom";
 import "network/public/interfaces/url_loader.mojom";
 
+// Interface implemented by content handlers. To avoid race conditions with
+// dropped requests, the implementation should keep a reference to the lifetime
+// of the app (by holding on to AppRefCount). Each application started by
+// StartApplication should call the callback given by that method on
+// destruction. When the owner in the shell notices this, it will destroy the
+// interface pointer, which should cause the strongly-bound ContentHandler
+// implementation to self destruct and release the app reference.
 interface ContentHandler {
-  StartApplication(Application& application, URLResponse response);
+  // The callback should be called when the application is destructed.
+  StartApplication(Application& application, URLResponse response) => ();
 };