For the out-of-process NativeRunner, make sure that Start()'s callback always gets called.

shell/app_child_process_host.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SHELL_APP_CHILD_PROCESS_HOST_H_
 #define SHELL_APP_CHILD_PROCESS_HOST_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "mojo/edk/embedder/channel_info_forward.h"
+#include "mojo/public/cpp/bindings/error_handler.h"
 #include "shell/app_child_process.mojom.h"
 #include "shell/child_process_host.h"
 
 namespace mojo {
 namespace shell {
 
 // A subclass of |ChildProcessHost| to host an app child process, which runs a
 // single app (loaded from the file system).
 //
 // Note: After |Start()|, |StartApp| must be called and this object must
 // remained alive until the |on_app_complete| callback is called.
-class AppChildProcessHost : public ChildProcessHost {
+class AppChildProcessHost : public ChildProcessHost, public ErrorHandler {
  public:
   explicit AppChildProcessHost(Context* context);
   ~AppChildProcessHost() override;
 
-  // See |AppChildController|:
+  // See |AppChildController::StartApp()| (this may only be called after
+  // |Start()|). Note, however, one difference: |on_app_complete| will *always*
+  // get called, even on connection error (or even if the child process failed
+  // to start at all).
   void StartApp(const String& app_path,
                 bool clean_app_path,
                 InterfaceRequest<Application> application_request,
                 const AppChildController::StartAppCallback& on_app_complete);
+  // See |AppChildController::ExitNow()|.
   void ExitNow(int32_t exit_code);
 
   // |ChildProcessHost| methods:
   void WillStart() override;
   void DidStart(bool success) override;
 
+  // |ErrorHandler| methods:
+  void OnConnectionError() override;
+
  private:
   // Callback for |embedder::CreateChannel()|.
   void DidCreateChannel(embedder::ChannelInfo* channel_info);
 
   void AppCompleted(int32_t result);
 
   AppChildControllerPtr controller_;
   embedder::ChannelInfo* channel_info_;
   AppChildController::StartAppCallback on_app_complete_;
 
   DISALLOW_COPY_AND_ASSIGN(AppChildProcessHost);
 };
 
 }  // namespace shell
 }  // namespace mojo
 
 #endif  // SHELL_APP_CHILD_PROCESS_HOST_H_