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: de016e52bd170d2d4f2344f9bf92d50478b649e0@23969 0039d316-1c4b-4281-b951-d872f2087c98

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