Oilpan: fix build after r200912.

TBR=oilpan-reviews
BUG=492204
NOTRY=true

Review URL: https://codereview.chromium.org/1298253004

git-svn-id: svn://svn.chromium.org/blink/trunk@200925 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/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;
 };