Mark http/tests/xmlhttprequest/event-listener-gc.html as flaky

This patch marks http/tests/xmlhttprequest/event-listener-gc.html as
flaky on all platforms. There is no obvious start of the flakiness.

TBR=fmalita
NOTRY=true
NOTREECHECKS=true
BUG=341331

Review URL: https://codereview.chromium.org/146143004

git-svn-id: svn://svn.chromium.org/blink/trunk@166576 bbb929c8-8fbe-4397-9dbb-9b2b20218538