- Adds SetRandomMmapSeed method.
- Removes random seed from OS::Initialize signature.

Bug: chromium:756050
Change-Id: I46ef6a92efb8b97541b90383d08776dd73a35a0c
Reviewed-on: https://chromium-review.googlesource.com/836907Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50280}