[ServiceWorker] Skip resolving the promise when the execution context has gone in FetchManager::Loader.

BUG=None
TEST=run_webkit_tests http/tests/serviceworker/

Review URL: https://codereview.chromium.org/536823003

git-svn-id: svn://svn.chromium.org/blink/trunk@181296 bbb929c8-8fbe-4397-9dbb-9b2b20218538