Fix the utility process launching.

This would cause a crash of the render process while loading
facebook.com since the utility process would try to use
/proc/self/exe on linux.

Bring back the kBrowserSubprocessPath switch to specify our
subprocess executable.

Change-Id: I4822d43f4a2b5ee86b941721da5ebb47d7a97c5d
Reviewed-by: Arvid Nilsson <anilsson@blackberry.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>