Destroy DeviceImpl when the underlying UsbDevice is disconnected.

device/usb/mojo/device_impl.cc

Index: device/usb/mojo/device_impl.cc
diff --git a/device/usb/mojo/device_impl.cc b/device/usb/mojo/device_impl.cc
index 0d1c64f6a2dd1336fe8b659ce9407633af9dbdf7..ae5f4bb53bef88d4f4324f13b163606e3df0f381 100644
--- a/device/usb/mojo/device_impl.cc
+++ b/device/usb/mojo/device_impl.cc
@@ -179,13 +179,17 @@ void OnIsochronousTransferOut(
 DeviceImpl::DeviceImpl(scoped_refptr<UsbDevice> device,
                        PermissionProviderPtr permission_provider,
                        mojo::InterfaceRequest<Device> request)
-    : binding_(this, std::move(request)),
-      device_(device),
+    : device_(device),
+      observer_(this),
       permission_provider_(std::move(permission_provider)),
+      binding_(this, std::move(request)),
       weak_factory_(this) {
-  // This object owns itself and will be destroyed if either the message pipe
-  // it is bound to is closed or the PermissionProvider it depends on is
-  // unavailable.
+  DCHECK(device_);
+  // This object owns itself and will be destroyed if,
+  //  * the device is disconnected,
+  //  * the message pipe it is bound to is closed, or
+  //  * the PermissionProvider it depends on is unavailable.
+  observer_.Add(device_.get());
   binding_.set_connection_error_handler([this]() { delete this; });
   permission_provider_.set_connection_error_handler([this]() { delete this; });
 }
@@ -463,5 +467,10 @@ void DeviceImpl::IsochronousTransferOut(
       base::Bind(&OnIsochronousTransferOut, base::Passed(&callback_ptr)));
 }
 
+void DeviceImpl::OnDeviceRemoved(scoped_refptr<UsbDevice> device) {
+  DCHECK_EQ(device_, device);
+  delete this;
+}
+
 }  // namespace usb
 }  // namespace device