Unofficial gardening. Marked 3 webaudio tests which recently started timing out as Slow to see if they pass.

TBR=
BUG=265059

Review URL: https://codereview.chromium.org/20598003

git-svn-id: svn://svn.chromium.org/blink/trunk@155054 bbb929c8-8fbe-4397-9dbb-9b2b20218538