Revert r23911, which switched from fork to vfork().
Revert r23911, which switched from fork to vfork().

Everything still worked correctly, but our performance bots didn't measure
any improvements.  Perhaps I just ended up measuring the dtrace overhead of
fork vs vfork.

BUG=19863

Review URL: http://codereview.chromium.org/174229


git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@23969 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser