Remove single observer assumption in WebUSBClient.

third_party/WebKit/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"
 
@@ skipping 22 matching lines @@
     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*) = 0;
+    // Adds an observer of device changes to the WebUSBClient.
+    virtual void addObserver(Observer*) = 0;
+
+    // Removes an observer of device changes from the WebUSBClient.
+    virtual void removeObserver(Observer*) = 0;
 };
 
 } // namespace blink
 
 #endif // WebUSBClient_h