Get rid of multiprocess_test's debug_on_start arguments.

(It was only ever given a "true" value once, and even that seemed dubious.)

R=phajdan.jr@chromium.org
TBR=darin@chromium.org, jeremy@chromium.org

Review URL: https://codereview.chromium.org/191483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255801