Mark two tests as flaky on release (besides debug)

BUG=418438
TEST=bots
TBR=lushnikov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/618443002

git-svn-id: svn://svn.chromium.org/blink/trunk@182856 bbb929c8-8fbe-4397-9dbb-9b2b20218538