DevTools: gardening: mark http/tests/inspector/tracing as flaky.

BUG=420008
TBR=yurys@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/654483003

git-svn-id: svn://svn.chromium.org/blink/trunk@183592 bbb929c8-8fbe-4397-9dbb-9b2b20218538