Disable autodetection for XML documents

Unlabelled XML should be treated as UTF-8-encoded documents.
This CL disables autodetection for XML, and puts the suppressed
layout tests back on.

BUG=597488

Review-Url: https://codereview.chromium.org/2320433002
Cr-Commit-Position: refs/heads/master@{#416848}
2 files changed