chromium: Fix userns sandbox patch for version 36.

This fixes build for version 36, which i accidentally broke in commit
f6e31fadd80486cdd68e3ad3d7ae888aa81400b9.

The reason this happened, was that my Hydra didn't pick up the latest
commit and I actually tested and built the parent commit instead of the
update commit.

So, this commit is the real "builds fine, tested" for all channels.

Also, the sandbox client initalization has moved into
setuid_sandbox_client.cc, so we need to move the lookup of the
CHROMIUM_SANDBOX_BINARY_PATH environment variable there.

AMEND ME AND SIGN OFF - DO NOT MERGE, HYDRA-PICKUP!