Oilpan: fix build after r200912.

Source/modules/webusb/USB.h

Index: Source/modules/webusb/USB.h
diff --git a/Source/modules/webusb/USB.h b/Source/modules/webusb/USB.h
index 5d2d60ca1894b3c65f34f596865955afa2d624f1..d8c252b5b286e9925602335842c18f7ab5571a3f 100644
--- a/Source/modules/webusb/USB.h
+++ b/Source/modules/webusb/USB.h
@@ -21,15 +21,12 @@ class USB final
     : public RefCountedGarbageCollectedEventTargetWithInlineData<USB> {
     DEFINE_WRAPPERTYPEINFO();
     REFCOUNTED_GARBAGE_COLLECTED_EVENT_TARGET(USB);
-    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(USB);
 public:
     static USB* create(LocalFrame& frame)
     {
         return new USB(frame);
     }
 
-    explicit USB(LocalFrame& frame);
-
     // USB.idl
     ScriptPromise getDevices(ScriptState*);
     ScriptPromise requestDevice(ScriptState*, const USBDeviceRequestOptions&);
@@ -43,6 +40,8 @@ public:
     DECLARE_VIRTUAL_TRACE();
 
 private:
+    explicit USB(LocalFrame& frame);
+
     RawPtrWillBeMember<USBController> m_controller;
 };