webgui: remove workaround for headless chrome

In normall condition headless chrome immediately exits after page and
all scripts are loaded. To keep it running, special flag was used
enabling debugging mode and external ports. 

Much better solution - append special script to the page which never
loads. Chrome will keep running forever - until we kill it.

gui/webdisplay/src/TWebWindowsManager.cxx

@@ -564,12 +564,12 @@ bool ROOT::Experimental::TWebWindowsManager::Show(ROOT::Experimental::TWebWindow
       is_chrome = prog.Length()>0;
 #ifdef _MSC_VER
       if (win.IsBatchMode())
-         exec = gEnv->GetValue("WebGui.ChromeBatch", "fork: --headless --disable-gpu --remote-debugging-port=$port $url");
+         exec = gEnv->GetValue("WebGui.ChromeBatch", "fork: --headless --disable-gpu $url");
       else
          exec = gEnv->GetValue("WebGui.ChromeInteractive", "$prog --window-size=$width,$height --app=$url");
 #else
       if (win.IsBatchMode())
-         exec = gEnv->GetValue("WebGui.ChromeBatch", "fork:--headless --disable-gpu --disable-webgl --remote-debugging-pipe $url");
+         exec = gEnv->GetValue("WebGui.ChromeBatch", "fork:--headless $url");
       else
          exec = gEnv->GetValue("WebGui.ChromeInteractive", "$prog --window-size=$width,$height --app=\'$url\' &");
 #endif
@@ -685,8 +685,6 @@ bool ROOT::Experimental::TWebWindowsManager::Show(ROOT::Experimental::TWebWindow
 
    addr = fAddr + addr;
 
-   int port = gEnv->GetValue("WebGui.HeadlessPort", 9222);
-   exec.ReplaceAll("$port", std::to_string(port).c_str());
    exec.ReplaceAll("$url", addr.c_str());
    exec.ReplaceAll("$width", swidth.c_str());
    exec.ReplaceAll("$height", sheight.c_str());