Remove dependency between PageAllocator and platform.

We move away from cryptographicallyRandomValues() and onto a simple built-in
random generator.

(This is a subset of previously reviewed
https://codereview.chromium.org/25644002/)

BUG=270531
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/26812002

git-svn-id: svn://svn.chromium.org/blink/trunk@159327 bbb929c8-8fbe-4397-9dbb-9b2b20218538