[DevTools] Frontend should be notified when user rejects to save file.

Currently still open dialog is indistinguishable from canceled one.

BUG=284243

Review URL: https://codereview.chromium.org/86023002

git-svn-id: svn://svn.chromium.org/blink/trunk@162804 bbb929c8-8fbe-4397-9dbb-9b2b20218538