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}
HeapVector<VRLayer> VRDisplay::getLayers() {
  HeapVector<VRLayer> layers;

  if (is_presenting_) {
    layers.push_back(layer_);
  }

  return layers;
}

HeapVector<VRLayer> VRDisplay::getLayers() {
  HeapVector<VRLayer> layers;

  if (is_presenting_) {
    layers.push_back(layer_);
  }

  return layers;
}
