Refactor MIME type related methods in XHR for readability

- Give more descriptive names to MIME type related methods
- Split responseMIMEType() into one without fallback to text/xml and one
  with it as this fallback is requird only a few places
- Add some FIXMEs to address issues found during this refactoring later

BUG=none

Review URL: https://codereview.chromium.org/450103003

git-svn-id: svn://svn.chromium.org/blink/trunk@179950 bbb929c8-8fbe-4397-9dbb-9b2b20218538