Allow VRDisplay to manage it's own rAF callbacks.

This will be necessary when we begin firing VR rAFs at a different rate than window.rAF, but here is used to prevent misuse of submitFrame, which needs to happen in a VR-specific rAF in order to ensure a good experience across the board.

BUG=389343

Review-Url: https://codereview.chromium.org/2413903002
Cr-Commit-Position: refs/heads/master@{#427496}
3 files changed