Issue #1652, #3456 - Replace 's_asyncWebBrowser2' with thread-safe facilities

src/plugin/PluginClass.h

Index: src/plugin/PluginClass.h
===================================================================
--- a/src/plugin/PluginClass.h
+++ b/src/plugin/PluginClass.h
@@ -28,7 +28,7 @@
 #define _CRTDBG_MAP_ALLOC
 #include <stdlib.h>
 #include <crtdbg.h>
-#include <set>
+#include <mutex>
 
 #include "NotificationMessage.h"
 
@@ -50,6 +50,7 @@
 {
 
   friend class CPluginTab;
+  friend class AnyCurrentSiteBrowser;
 
 private:
 
@@ -102,7 +103,6 @@
 public:
   HWND GetBrowserHWND() const;
   bool IsRootBrowser(IWebBrowser2*);
-
   static CPluginMimeFilterClient* s_mimeFilter;
 
 private:
@@ -135,7 +135,6 @@
   void Unadvise();
 
   void ShowStatusBar();
-  bool IsStatusBarEnabled();
 
   /**
    * A browser interface pointer to our site object
@@ -143,6 +142,11 @@
    * It's values are set and reset solely in SetSite().
    */
   CComPtr<IWebBrowser2> m_webBrowser2;
+  /**
+   * Synchronizes SetSite() with references to our site pointer.
+   */
+  std::mutex siteGuard;
+
   HWND m_hBrowserWnd;
   HWND m_hTabWnd;
   HWND m_hStatusBarWnd;
@@ -175,13 +179,8 @@
   static bool s_isMainThreadDone;
 
   static HINSTANCE s_hUxtheme;
-  static std::set<CPluginClass*> s_instances;
   static CComAutoCriticalSection s_criticalSectionLocal;
   static CComAutoCriticalSection s_criticalSectionWindow;
-
-  // Async browser
-  static CComQIPtr<IWebBrowser2> s_asyncWebBrowser2;
-  static CComQIPtr<IWebBrowser2> GetAsyncBrowser();
 };
 
 OBJECT_ENTRY_AUTO(__uuidof(PluginClass), CPluginClass)