[FileAPI] Drop irrelevant ASSERT in FileWriter

The error code can be ABORT_ERR if a operation is aborted by the browser.

BUG=369525

Review URL: https://codereview.chromium.org/267253008

git-svn-id: svn://svn.chromium.org/blink/trunk@173620 bbb929c8-8fbe-4397-9dbb-9b2b20218538