Track USB alternate interface state in Blink.

third_party/WebKit/Source/modules/webusb/USBInterface.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/USBInterface.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "modules/webusb/USBAlternateInterface.h"
 #include "modules/webusb/USBConfiguration.h"
 #include "modules/webusb/USBDevice.h"
@@ skipping 23 matching lines @@
 {
     ASSERT(m_configurationIndex < m_device->info().configurations.size());
     ASSERT(m_interfaceIndex < m_device->info().configurations[m_configurationIndex].interfaces.size());
 }
 
 const WebUSBDeviceInfo::Interface& USBInterface::info() const
 {
     return m_device->info().configurations[m_configurationIndex].interfaces[m_interfaceIndex];
 }
 
+USBAlternateInterface* USBInterface::alternate() const
+{
+    if (m_device->isInterfaceClaimed(m_configurationIndex, m_interfaceIndex))
+        return USBAlternateInterface::create(this, m_device->selectedAlternateInterface(m_interfaceIndex));
+    return nullptr;
+}
+
 HeapVector<Member<USBAlternateInterface>> USBInterface::alternates() const
 {
     HeapVector<Member<USBAlternateInterface>> alternates;
     for (size_t i = 0; i < info().alternates.size(); ++i)
         alternates.append(USBAlternateInterface::create(this, i));
     return alternates;
 }
 
 uint8_t USBInterface::interfaceNumber() const
 {
     return info().interfaceNumber;
 }
 
 bool USBInterface::claimed() const
 {
     return m_device->isInterfaceClaimed(m_configurationIndex, m_interfaceIndex);
 }
 
 DEFINE_TRACE(USBInterface)
 {
     visitor->trace(m_device);
 }
 
 } // namespace blink