Fix for crash on quit w.r.t any web worker still running during quit. ( ...

...[CEF 2171] Crash on quit. #7683 )

This fixes the crash with web workers still running while quitting the
main Brackets app. CefQuitMessageLoop() needs to be called for the CEF
message loop to quit gracefully. This change involves removing handling
of WM_DESTROY and doing the same in ClientHandler::OnBeforeClose().