Add CRYPTO_pre_sandbox_init.

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>
diff --git a/include/openssl/crypto.h b/include/openssl/crypto.h
index e539bdb..0dc5373 100644
--- a/include/openssl/crypto.h
+++ b/include/openssl/crypto.h
@@ -63,6 +63,14 @@
 // success and zero on error.
 OPENSSL_EXPORT int BORINGSSL_self_test(void);
 
+// CRYPTO_pre_sandbox_init initializes the crypto library, pre-acquiring some
+// unusual resources to aid running in sandboxed environments. It is safe to
+// call this function multiple times and concurrently from multiple threads.
+//
+// For more details on using BoringSSL in a sandboxed environment, see
+// SANDBOXING.md in the source tree.
+OPENSSL_EXPORT void CRYPTO_pre_sandbox_init(void);
+
 
 // Deprecated functions.