Add an explicit dispose step for WebDevToolsAgentImpl.

Source/web/WebDevToolsAgentImpl.h

Index: Source/web/WebDevToolsAgentImpl.h
diff --git a/Source/web/WebDevToolsAgentImpl.h b/Source/web/WebDevToolsAgentImpl.h
index b96bb066bca8bfeecf99d8ab33aefcf8f2f7065c..a54c0bc098f5699742354b963cd96f48b38163c3 100644
--- a/Source/web/WebDevToolsAgentImpl.h
+++ b/Source/web/WebDevToolsAgentImpl.h
@@ -75,6 +75,7 @@ class WebDevToolsAgentImpl final
 public:
     WebDevToolsAgentImpl(WebViewImpl*, WebDevToolsAgentClient*);
     ~WebDevToolsAgentImpl() override;
+    void dispose();
     DECLARE_VIRTUAL_TRACE();
 
     void willBeDestroyed();
@@ -164,6 +165,9 @@ private:
     WebDevToolsAgentClient* m_client;
     WebViewImpl* m_webViewImpl;
     bool m_attached;
+#if ENABLE(ASSERT)
+    bool m_hasBeenDisposed;
+#endif
 
     RefPtrWillBeMember<InstrumentingAgents> m_instrumentingAgents;
     OwnPtrWillBeMember<InjectedScriptManager> m_injectedScriptManager;