[ServiceWorker] Catch up to a change in the Fetch spec

This change follows the change in Fech spec
https://github.com/whatwg/fetch/commit/540a828e4bff394c1b44abef9a94424cac3fbea1
https://github.com/whatwg/fetch/issues/9

In current code:
 var baseRequest = new Request('test.html', {headers: {'X-Test': 'test'}});
 var request = new Request(baseRequest, {mode: 'no-cors'});
 console.log(request.headers.get('X-Test'))  // -> 'test

Expected:
 'X-Test' header should be omitted in the constructor of |request| because the mode is 'no-cors'.

BUG=373120

Review URL: https://codereview.chromium.org/382453003

git-svn-id: svn://svn.chromium.org/blink/trunk@177799 bbb929c8-8fbe-4397-9dbb-9b2b20218538