Remove single observer assumption in WebUSBClient.

third_party/WebKit/Source/modules/webusb/USB.cpp

 // 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.
 
 #include "modules/webusb/USB.h"
 
 #include "bindings/core/v8/CallbackPromiseAdapter.h"
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "core/dom/DOMException.h"
@@ skipping 60 matching lines @@
         return devices;
     }
 };
 
 } // namespace
 
 USB::USB(LocalFrame& frame)
     : LocalFrameLifecycleObserver(&frame)
     , m_client(USBController::from(frame).client())
 {
+    ThreadState::current()->registerPreFinalizer(this);
     if (m_client)
-        m_client->setObserver(this);
+        m_client->addObserver(this);
 }
 
 USB::~USB()
 {
+}
+
+void USB::dispose()
+{
     if (m_client)
-        m_client->setObserver(nullptr);
+        m_client->removeObserver(this);

[haraken, 2016/04/01 01:36:06]

Shall we clear m_client here just in case? Then you can DCHECK(!m_client) in
USB's destructor.

 }
 
 ScriptPromise USB::getDevices(ScriptState* scriptState)
 {
     if (!m_client)
         return ScriptPromise::rejectWithDOMException(scriptState, DOMException::create(NotSupportedError));
 
     String errorMessage;
     if (!scriptState->getExecutionContext()->isSecureContext(errorMessage))
         return ScriptPromise::rejectWithDOMException(scriptState, DOMException::create(SecurityError, errorMessage));
@@ skipping 33 matching lines @@
 }
 
 const AtomicString& USB::interfaceName() const
 {
     return EventTargetNames::USB;
 }
 
 void USB::willDetachFrameHost()
 {
     if (m_client)
-        m_client->setObserver(nullptr);
+        m_client->removeObserver(this);
     m_client = nullptr;
 }
 
 void USB::onDeviceConnected(WebPassOwnPtr<WebUSBDevice> device)
 {
     dispatchEvent(USBConnectionEvent::create(EventTypeNames::connect, USBDevice::create(device.release(), getExecutionContext())));
 }
 
 void USB::onDeviceDisconnected(WebPassOwnPtr<WebUSBDevice> device)
 {
     dispatchEvent(USBConnectionEvent::create(EventTypeNames::disconnect, USBDevice::create(device.release(), getExecutionContext())));
 }
 
 DEFINE_TRACE(USB)
 {
     RefCountedGarbageCollectedEventTargetWithInlineData<USB>::trace(visitor);
     LocalFrameLifecycleObserver::trace(visitor);
 }
 
 } // namespace blink