Allow using a fixed version with wxWebViewEdge

A fixed version may be distributed with the application
and wxWebViewEdge::MSWSetBrowserExecutableDir() allows
it's usage with wxWebViewEdge.

include/wx/msw/private/webview_edge.h

@@ -66,6 +66,7 @@ public:
     ICoreWebView2Settings* GetSettings();
 
     static wxDynamicLibrary ms_loaderDll;
+    static wxString ms_browserExecutableDir;
 
     static bool Initialize();