Implement USB connection events through the WebUSB client interface.

public/platform/modules/webusb/WebUSBClient.h

 // Copyright 2015 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 WebUSBClient_h
 #define WebUSBClient_h
 
 #include "public/platform/WebCallbacks.h"
 #include "public/platform/WebVector.h"
 
 namespace blink {
 
 class WebUSBDevice;
 struct WebUSBDeviceRequestOptions;
 struct WebUSBError;
 
 using WebUSBClientGetDevicesCallbacks = WebCallbacks<WebPassOwnPtr<WebVector<WebUSBDevice*>>, const WebUSBError&>;
 using WebUSBClientRequestDeviceCallbacks = WebCallbacks<WebPassOwnPtr<WebUSBDevice>, const WebUSBError&>;
 
 class WebUSBClient {
 public:
+    class Observer {
+    public:
+        virtual ~Observer() { }
+
+        // Called when a device is connected to the system.
+        virtual void onDeviceConnected(WebPassOwnPtr<WebUSBDevice>) = 0;
+
+        // Called when a device is disconnected from the system.
+        virtual void onDeviceDisconnected(WebPassOwnPtr<WebUSBDevice>) = 0;
+    };
+
     virtual ~WebUSBClient() { }
 
     // Enumerates available devices.
     // Ownership of the WebUSBClientGetDevicesCallbacks is transferred to the client.
     virtual void getDevices(WebUSBClientGetDevicesCallbacks*) = 0;
 
     // Requests access to a device.
     // Ownership of the WebUSBClientRequestDeviceCallbacks is transferred to the client.
     virtual void requestDevice(const WebUSBDeviceRequestOptions&, WebUSBClientRequestDeviceCallbacks*) = 0;
+
+    // Sets the observer of device changes through the WebUSBClient.
+    virtual void setObserver(Observer*) { }
 };
 
 } // namespace blink
 
 #endif // WebUSBClient_h