[ServiceWorker] Skip resolving the promise when the execution context has gone in FetchBodyStream.

This will fix the crash of the LayoutTest (request.html).
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showLargeExpectations=true&tests=http%2Ftests%2Fserviceworker%2Frequest.html


BUG=409755,402387
TEST=run_webkit_tests http/tests/serviceworker/request.html

Review URL: https://codereview.chromium.org/531363002

git-svn-id: svn://svn.chromium.org/blink/trunk@181287 bbb929c8-8fbe-4397-9dbb-9b2b20218538