[ServiceWorker] Add WebServiceWorkerRequest::setBlob() to support body of FetchEvent

BUG=402387

Review URL: https://codereview.chromium.org/483603003

git-svn-id: svn://svn.chromium.org/blink/trunk@180430 bbb929c8-8fbe-4397-9dbb-9b2b20218538