Check --utility-enable-mdns before local_discovery::ServiceDiscoveryMessageHandler::PreSandboxStartup call.

BUG=350518

Review URL: https://codereview.chromium.org/191513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255829 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255829