DevTools: add Timeline.start command use counter

This is going to be used for tracking usage of the deprecated Timeline implementation.

BUG=428701

Review URL: https://codereview.chromium.org/695453002

git-svn-id: svn://svn.chromium.org/blink/trunk@184653 bbb929c8-8fbe-4397-9dbb-9b2b20218538