[ChromeOS] Add option to set controlling terminal when launching process.

This is needed to expose crosh process to hterm (if pseudo terminal used for
communication between terminalPrivateApi and crosh process is not controlling, job
control in the crosh process won't work, and it won't be able to e.g. process signals).

TEST=None
BUG=chromium-os:23273

Review URL: https://chromiumcodereview.appspot.com/9176013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@118585 0039d316-1c4b-4281-b951-d872f2087c98

• base/process_util.h[diff]
• base/process_util_posix.cc[diff]