Collect mutex and data protected by it in a single struct

Also use critical section instead of a mutex, as this is more efficient
under MSW.

Main purpose of this commit is to make it clear that this mutex/critical
section is only used together with the data from the same struct.

No real changes.

include/wx/private/webrequest_curl.h

@@ -145,8 +145,13 @@ private:
     wxMutex m_mutex;
     wxCondition m_condition;
     bool m_shuttingDown;
-    wxMutex m_cancelledMutex;
-    wxVector< wxObjectDataPtr<wxWebRequestCURL> > m_cancelledRequests;
+
+    // MT-safe vector of requests for which Cancel() was called.
+    struct CancelledData
+    {
+        wxCriticalSection cs;
+        wxVector< wxObjectDataPtr<wxWebRequestCURL> > requests;
+    } m_cancelled;
 
     static int ms_activeSessions;