Mark file-writer-events-shared-worker.html as flaky on win7 debug

BUG=321857
TEST=
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/79363005

git-svn-id: svn://svn.chromium.org/blink/trunk@162426 bbb929c8-8fbe-4397-9dbb-9b2b20218538