DevTools: enable tracing instantly upon getting Tracing.start on back-end

...so that the metadata we emit is not lost. Also, add a test for tracing
session id.

BUG=361045

Review URL: https://codereview.chromium.org/264583005

git-svn-id: svn://svn.chromium.org/blink/trunk@173300 bbb929c8-8fbe-4397-9dbb-9b2b20218538