Stop observing worker context after WorkerFileWriterCallbacksBridge gets shutdown

BUG=258949
TEST=fast/filesystem/workers/*
R=ericu@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19587004

git-svn-id: svn://svn.chromium.org/blink/trunk@154706 bbb929c8-8fbe-4397-9dbb-9b2b20218538