Destroy DeviceImpl when the underlying UsbDevice is disconnected.

device/usb/usb_device.cc

Index: device/usb/usb_device.cc
diff --git a/device/usb/usb_device.cc b/device/usb/usb_device.cc
index d7ab38bde3d65429eedb2e0a7a259fc8622d7c24..d34e3e0d0f4f21edcfe29c9e0b5befc9fb924abd 100644
--- a/device/usb/usb_device.cc
+++ b/device/usb/usb_device.cc
@@ -9,6 +9,10 @@
 
 namespace device {
 
+UsbDevice::Observer::~Observer() {}
+
+void UsbDevice::Observer::OnDeviceRemoved(scoped_refptr<UsbDevice> device) {}
+
 UsbDevice::UsbDevice(uint16_t vendor_id,
                      uint16_t product_id,
                      const base::string16& manufacturer_string,
@@ -30,4 +34,16 @@ void UsbDevice::CheckUsbAccess(const ResultCallback& callback) {
   callback.Run(true);
 }
 
+void UsbDevice::AddObserver(Observer* observer) {
+  observer_list_.AddObserver(observer);
+}
+
+void UsbDevice::RemoveObserver(Observer* observer) {
+  observer_list_.RemoveObserver(observer);
+}
+
+void UsbDevice::NotifyDeviceRemoved() {
+  FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceRemoved(this));
+}
+
 }  // namespace device