Stop recording CPU profiles when Profiler agent is disabled

Also Profiler.start command will fail unless Profiler was enabled.

BUG=327298
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/116273003

git-svn-id: svn://svn.chromium.org/blink/trunk@163962 bbb929c8-8fbe-4397-9dbb-9b2b20218538