[Fetch] Always use utf-8 for decoding in text()

In the spec, Response.text() should run utf-8 decode [1][2], i.e. always use
utf-8. But in the implementation, TextResourceDecoder used by Response.text()
decodes the input as utf-16/32 if it contains utf-16/32 BOMs.

This CL
- Adds TextResourceDecoder::createAlwaysUseUTF8ForText() and
  AlwaysUseUTF8ForText that corresponds to utf-8 decode in the spec [2].
- Fixes Response.text() by using CheckForOnlyUTF8BOM in FetchDataLoaderAsString.
- Adds layout tests for Fetch API and XHRs
  (but doesn't change the behavior of XHRs).

[1] https://fetch.spec.whatwg.org/#concept-body-package-data
[2] https://encoding.spec.whatwg.org/#utf-8-decode

BUG=557510

Review URL: https://codereview.chromium.org/1470893002

Cr-Commit-Position: refs/heads/master@{#364978}
12 files changed