Don't expose XMLHttpRequest to Service Workers.

Service Workers should use fetch() instead of XHR. This also deletes
the fetch polyfill, which was built on top of XHR.

BUG=395931

Review URL: https://codereview.chromium.org/408143002

git-svn-id: svn://svn.chromium.org/blink/trunk@178829 bbb929c8-8fbe-4397-9dbb-9b2b20218538