DevTools: [Timeline] do not collect "devtools" category because we do not use it any more.

BUG=361045

Review URL: https://codereview.chromium.org/365103002

git-svn-id: svn://svn.chromium.org/blink/trunk@177485 bbb929c8-8fbe-4397-9dbb-9b2b20218538