DevTools: wrong event was used as an indicator of finished write operation.

FileWriter calls onwrite as notification about the progress of writing.
So If we call write again from the onwrite event handler FileWriter fails to do that.
Actually we need to use onwriteend callback.

BUG=423843

Review URL: https://codereview.chromium.org/661063002

git-svn-id: svn://svn.chromium.org/blink/trunk@183886 bbb929c8-8fbe-4397-9dbb-9b2b20218538