[XHR] Use correct TextResourceDecoderOption

Blink enabled HTML / XML charset detection for HTML-ish / XML-ish
responses even when response type is text. This CL fixes the issue.

This CL removes some layout tests. The behavior is tested by
wpt/XMLHttpRequest/responsetext-decoding.htm,
wpt/XMLHttpRequest/responsedocument-decoding.htm,
wpt/XMLHttpRequest/overridemimetype-open-state-force-utf-8.htm and
wpt/XMLHttpRequest/overridemimetype-open-state-force-xml.

Bug: 695045
Change-Id: I82b223dd85907350ce1e7787969bd1d4de85aa3a
Reviewed-on: https://chromium-review.googlesource.com/701899
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507915}
6 files changed