WebVR: fix initial vsync

Applications sometimes use window.rAF while not presenting, then switch to
vrDisplay.rAF after presentation starts. Depending on the animation loop's
timing, this can cause a race condition where presentation has been started
but there's no vrDisplay.rAF pending yet. Ensure there's at least vsync
being processed after presentation starts so that a queued window.rAF
can run and schedule a vrDisplay.rAF.

BUG=711789

Review-Url: https://codereview.chromium.org/2848483003
Cr-Commit-Position: refs/heads/master@{#468167}
void VRDisplay::OnChanged(device::mojom::blink::VRDisplayInfoPtr display) {
  Update(display);
}

void VRDisplay::OnChanged(device::mojom::blink::VRDisplayInfoPtr display) {
  Update(display);
}
