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();
 

include/wx/msw/webview_edge.h

@@ -122,6 +122,8 @@ public:
 
     virtual void* GetNativeBackend() const wxOVERRIDE;
 
+    static void MSWSetBrowserExecutableDir(const wxString& path);
+
 protected:
     virtual void DoSetPage(const wxString& html, const wxString& baseUrl) wxOVERRIDE;