Forbid setting responseType on all sync XHRs.

Rather than only doing it for http(s) URLs. This matches the XHR2 spec and
Firefox. This also avoids having to support the downloadToFile + synchronous
load combination in the resource loader stack.

Update tests according and add an assertion.

BUG=354188

Review URL: https://codereview.chromium.org/206223005

git-svn-id: svn://svn.chromium.org/blink/trunk@169702 bbb929c8-8fbe-4397-9dbb-9b2b20218538