DevTools: tracing tests are slow.

BUG=420008
TBR= yurys@chromium.org, caseq@chromium.org, alph@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/623943007

git-svn-id: svn://svn.chromium.org/blink/trunk@183265 bbb929c8-8fbe-4397-9dbb-9b2b20218538