Implement WebUSBClient::setObserver in WebUSBClientImpl.

content/renderer/usb/web_usb_client_impl.h

Index: content/renderer/usb/web_usb_client_impl.h
diff --git a/content/renderer/usb/web_usb_client_impl.h b/content/renderer/usb/web_usb_client_impl.h
index 8aca79493a4a213c4882f9dd0870d76243134eae..dc2d1069c7b37c865b5569c0818b1b6817d37754 100644
--- a/content/renderer/usb/web_usb_client_impl.h
+++ b/content/renderer/usb/web_usb_client_impl.h
@@ -24,9 +24,14 @@ class WebUSBClientImpl : public blink::WebUSBClient {
   void requestDevice(
       const blink::WebUSBDeviceRequestOptions& options,
       blink::WebUSBClientRequestDeviceCallbacks* callbacks) override;
+  void setObserver(Observer* observer) override;
+
+  void OnDeviceChangeNotification(
+      device::usb::DeviceChangeNotificationPtr notification);
 
   mojo::ServiceProviderPtr device_services_;
   device::usb::DeviceManagerPtr device_manager_;
+  Observer* observer_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(WebUSBClientImpl);
 };