Move PRNG from BWE test framework to webrtc/test.

BUG=

Review URL: https://codereview.webrtc.org/1404953002

Cr-Original-Commit-Position: refs/heads/master@{#10285}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5bdddf91d300299a8d40f153b842dba61db58c60
9 files changed