Add some wxLogTrace() calls to wxWebRequestURLSession code

This is helpful when trying to understand what is going on, especially
because CFNETWORK_DIAGNOSTICS, which is supposed to do the same thing at
native level, doesn't seem to work (at least under 10.14).

include/wx/private/webrequest.h

@@ -21,6 +21,9 @@ WX_DECLARE_STRING_HASH_MAP(wxString, wxWebRequestHeaderMap);
 // Default buffer size when a fixed-size buffer must be used.
 const int wxWEBREQUEST_BUFFER_SIZE = 64 * 1024;
 
+// Trace mask used for the messages in wxWebRequest code.
+#define wxTRACE_WEBREQUEST "webrequest"
+
 // ----------------------------------------------------------------------------
 // wxWebAuthChallengeImpl
 // ----------------------------------------------------------------------------