Make flow event traces disabled-by-default

Currently all flow events are in the category "toplevel.flow", they originated from "ipc" and "task" categories.

BUG=338427

Review URL: https://codereview.chromium.org/148173011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249450 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249450