yhirano, could you please investigate this?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/47e4fc53e6d68c0a788fcc26de598b9e3848033f

commit 47e4fc53e6d68c0a788fcc26de598b9e3848033f
Author: Yutaka Hirano <yhirano@chromium.org>
Date: Wed Oct 11 07:03:53 2017

[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}
[delete] https://crrev.com/30dd3b61cd5b0d467abf38049a7847898402fbec/third_party/WebKit/LayoutTests/external/wpt/XMLHttpRequest/responsetext-decoding-expected.txt
[delete] https://crrev.com/30dd3b61cd5b0d467abf38049a7847898402fbec/third_party/WebKit/LayoutTests/fast/xmlhttprequest/xmlhttprequest-html-response-encoding-expected.txt
[delete] https://crrev.com/30dd3b61cd5b0d467abf38049a7847898402fbec/third_party/WebKit/LayoutTests/fast/xmlhttprequest/xmlhttprequest-html-response-encoding.html
[delete] https://crrev.com/30dd3b61cd5b0d467abf38049a7847898402fbec/third_party/WebKit/LayoutTests/http/tests/xmlhttprequest/response-encoding-expected.txt
[delete] https://crrev.com/30dd3b61cd5b0d467abf38049a7847898402fbec/third_party/WebKit/LayoutTests/http/tests/xmlhttprequest/response-encoding.html
[modify] https://crrev.com/47e4fc53e6d68c0a788fcc26de598b9e3848033f/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp