Add a new frame generator that cycles through randomly generated slides.

Like YuvFileGenerator, this also updates the display with a new slide on every Nth frame, but it generates the slides itself instead of reading them from files.

BUG=webrtc:8138

Review-Url: https://codereview.webrtc.org/3003193002
Cr-Original-Commit-Position: refs/heads/master@{#19585}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 579de6faef60ebe6e60a795041fecb92569d4f72
9 files changed