WinForms: don't report size changed when Size is 0x0.

This occurs when the control is on a modeless Form and it is minimized.
Same guard is in cefclient.
Merge pull request #512 from numbersint/fixes/winforms-minimize-fix

WinForms: don't report size changed when Size is 0x0.
Merge remote-tracking branch 'remotes/upstream/master' into feature/winforms-tabbed