Convert usages of mojo::ErrorHandler in //device to callbacks.

device/devices_app/devices_app.h

Index: device/devices_app/devices_app.h
diff --git a/device/devices_app/devices_app.h b/device/devices_app/devices_app.h
index eda8d4c98001e3505aaaa1f7e84edeab3a56229d..bbeb4bf98c03cd115659257043bb5e4bd24847b3 100644
--- a/device/devices_app/devices_app.h
+++ b/device/devices_app/devices_app.h
@@ -28,8 +28,7 @@ class DeviceManager;
 }
 
 class DevicesApp : public mojo::ApplicationDelegate,
-                   public mojo::InterfaceFactory<usb::DeviceManager>,
-                   public mojo::ErrorHandler {
+                   public mojo::InterfaceFactory<usb::DeviceManager> {
  public:
   explicit DevicesApp(
       scoped_refptr<base::SequencedTaskRunner> service_task_runner);
@@ -48,8 +47,8 @@ class DevicesApp : public mojo::ApplicationDelegate,
   void Create(mojo::ApplicationConnection* connection,
               mojo::InterfaceRequest<usb::DeviceManager> request) override;
 
-  // mojo::ErrorHandler:
-  void OnConnectionError() override;
+  // Mojo error handler to track device manager count.
+  void OnConnectionError();
 
   // Sets the app for destruction after a period of idle time. If any top-level
   // services (e.g. usb::DeviceManager) are bound before the timeout elapses,