Oilpan: tidy up finalization of InspectorEmulationAgent following r192106.

Source/web/InspectorEmulationAgent.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 InspectorEmulationAgent_h
 #define InspectorEmulationAgent_h
 
 #include "core/InspectorFrontend.h"
 #include "core/inspector/InspectorBaseAgent.h"
 
@@ skipping 14 matching lines @@
     // InspectorBackendDispatcher::EmulationCommandHandler implementation.
     void resetScrollAndPageScaleFactor(ErrorString*) override;
     void setPageScaleFactor(ErrorString*, double pageScaleFactor) override;
     void setScriptExecutionDisabled(ErrorString*, bool) override;
     void setTouchEmulationEnabled(ErrorString*, bool enabled, const String* configuration) override;
     void setEmulatedMedia(ErrorString*, const String&) override;
 
     // InspectorBaseAgent overrides.
     void disable(ErrorString*) override;
     void restore() override;
+    void discardAgent() override;
     void didCommitLoadForLocalFrame(LocalFrame*) override;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit InspectorEmulationAgent(WebViewImpl*);
 
     WebViewImpl* m_webViewImpl;
 };
 
 
 } // namespace blink
 
 
 #endif // !defined(InspectorEmulationAgent_h)