The intent is to replace the logic in [0] and allows Chromium to set up
the MADV_WIPEONFORK page without increasing sandbox syscall surface.
From there we can remove RAND_set_urandom_fd and trim a bit of
complexity from the PRNG logic.

[0] https://source.chromium.org/chromium/chromium/src/+/master:content/app/content_main_runner_impl.cc;l=333-341;drc=975850fa57e140ec696114477e9416a19f06d29f

Change-Id: I9b679e15da551a10302389556c6c77d192be662a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/41326


Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>