Implement USB connection events through the WebUSB client interface.

Source/modules/webusb/USB.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 USB_h
 #define USB_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/events/EventTarget.h"
 #include "modules/webusb/USBController.h"
 #include "platform/heap/Handle.h"
+#include "public/platform/modules/webusb/WebUSBClient.h"
 
 namespace blink {
 
 class LocalFrame;
 class ScriptState;
 class USBDeviceRequestOptions;
+class WebUSBDevice;
 
 class USB final
-    : public RefCountedGarbageCollectedEventTargetWithInlineData<USB> {
+    : public RefCountedGarbageCollectedEventTargetWithInlineData<USB>
+    , public WebUSBClient::Observer {
     DEFINE_WRAPPERTYPEINFO();
     REFCOUNTED_GARBAGE_COLLECTED_EVENT_TARGET(USB);
 public:
     static USB* create(LocalFrame& frame)
     {
         return new USB(frame);
     }
 
+    ~USB() override;
+
     // USB.idl
     ScriptPromise getDevices(ScriptState*);
     ScriptPromise requestDevice(ScriptState*, const USBDeviceRequestOptions&);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(connect);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(disconnect);
 
     // EventTarget overrides.
     ExecutionContext* executionContext() const override;
     const AtomicString& interfaceName() const override;
 
+    // WebUSBClient::Observer overrides.
+    void onDeviceConnected(WebPassOwnPtr<WebUSBDevice>) override;
+    void onDeviceDisconnected(WebPassOwnPtr<WebUSBDevice>) override;
+
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit USB(LocalFrame& frame);
 
     RawPtrWillBeMember<USBController> m_controller;
 };
 
 } // namespace blink
 
 #endif // USB_h