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().
Don't limit `CefQuitMessageLoop()` call to Windows only. CefClient test app calls it on other platforms and we should do the same thing on quit.
Merge pull request #490 from adobe/prashant_2171_crash_fix

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