Landing for Julien Tinnes, jln@google.com:

Landing for Julien Tinnes, jln@google.com:

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chroot to /proc instead of /tmp. This gets rid of a lot of unnecessary
complexity and fixes a race condition.

(Original idea from Markus)

The chroot helper will chroot to /proc/self/fdinfo (or /proc/self/fd). This is
pretty safe because access to this directory is protected by the ptrace() check
in the kernel and the helper is privileged.

Moreover, as soon as the helper _exit() and becomes a zombie, the directory
will be empty. Zygote should wait() for us to make everything deterministric.

We also export SBX_HELPER_PID so that Zygote can specifically wait for the
helper. 
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BUG=76542
R=markus,agl
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=79618