[Telemetry] Fix flake in netsim.

This is based on https://codereview.chromium.org/112103004/ , except that on my
Ubuntu workstation, I had to completely disable network manager to get rid of
the flake.

BUG=338537
R=szym@chromium.org

Review URL: https://codereview.chromium.org/144483008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248330 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248330