Adds a way to determine id of content handler that created app

mojo/application/public/interfaces/shell.mojom

Index: mojo/application/public/interfaces/shell.mojom
diff --git a/mojo/application/public/interfaces/shell.mojom b/mojo/application/public/interfaces/shell.mojom
index 3a313dd485148180697db538c8f538cc7fd68ebd..acec2c6d93c3645053477bdbf75a9d233ae45a0a 100644
--- a/mojo/application/public/interfaces/shell.mojom
+++ b/mojo/application/public/interfaces/shell.mojom
@@ -24,6 +24,9 @@ struct CapabilityFilter {
 // An interface through which a Mojo application may communicate with the Mojo
 // system and request connections to other applications.
 interface Shell {
+  // Used to indicate the app was not launched by a content handler.
+  const uint32 kInvalidContentHandlerID = 0;
+
   // Establishes a connection with another application ("target application")
   // (located at |request->url|) through which the calling application and the
   // target application may request services from one another.
@@ -56,10 +59,16 @@ interface Shell {
   // CapabilityFilter above. Note also that this parameter may be NULL, which
   // has the same meaning as allowing the target application to connect to
   // any application and service.
+  //
+  // If the connection to |application_url| involves a content handler, then
+  // |content_handler_id| is the id of the deepest content handler used to
+  // establish the connection to |application_url|. If no content handler is
+  // used |content_handler_id| is kInvalidContentHandlerID.
+  // TODO(beng): determine if we need to expose the target application id also.
   ConnectToApplication(URLRequest application_url,
                        ServiceProvider&? services,
                        ServiceProvider? exposed_services,
-                       CapabilityFilter? filter);
+                       CapabilityFilter? filter) => (uint32 content_handler_id);
 
   // When there are no more instantiated services in an application, it should
   // start its shutdown process by calling this method. Additionally, it should