Fix some build warnings

private field 'm_dwCookie' is not used
'return' will never be executed
result of comparison of unsigned enum expression < 0 is always false
'FlushDC' overrides a member function but is not marked 'override'
potentially uninitialized local variable 'bound' used

include/wx/msw/webview_ie.h

@@ -157,7 +157,6 @@ private:
     wxIEContainer* m_container;
     wxAutomationObject m_ie;
     IWebBrowser2* m_webBrowser;
-    DWORD m_dwCookie;
     wxCOMPtr<DocHostUIHandler> m_uiHandler;
 
     //We store the current zoom type;