Only prime NSS in the sandbox for NSS ports.
This code, if all goes well, can actually be deleted now. But leave it there as
ifdefs for now case we still need to revert the chimera.
Now that no calls to InitNSSSafely and friends are built in USE_OPENSSL ports,
the LoadNSSLibraries calls and /dev/urandom fopen override may be removed. They
were only added to support NSS in the sandbox.
BUG=506323
Review URL: https://codereview.chromium.org/
1274483002
Cr-Commit-Position: refs/heads/master@{#342846}