[telemetry] Enable flow events by default in telemetry when tracing.

BUG=

Review URL: https://codereview.chromium.org/168823005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251971 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251971