Don't use start command when starting native messaging hosts.

start calls 'cmd.exe /k' internally which keeps cmd running after
host process exits.

BUG=298304
R=mpcomplete@chromium.org, rvargas@chromium.org

Review URL: https://codereview.chromium.org/24646002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225300 0039d316-1c4b-4281-b951-d872f2087c98