HTTP method name normalization should not be implemented in XMLHttpRequest.

XMLHttpRequest::uppercaseKnownHTTPMethod implemented method normalization that
was once specified in the XMLHttpRequest spec. Because it is now specified in
the fetch spec, let's implement it in FetchUtils.

BUG=426816

Review URL: https://codereview.chromium.org/680513002

git-svn-id: svn://svn.chromium.org/blink/trunk@184432 bbb929c8-8fbe-4397-9dbb-9b2b20218538