Change XSS Auditor report's MIME type.

`application/json` might be dangerous to send to servers who aren't
expecting it. Let's move to a MIME type specific to this reporting
structure.

BUG=691726

Review-Url: https://codereview.chromium.org/2717463006
Cr-Commit-Position: refs/heads/master@{#453086}
4 files changed