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}
DEFINE_TRACE(VRDisplay) {
  EventTargetWithInlineData::Trace(visitor);
  ContextLifecycleObserver::Trace(visitor);
  visitor->Trace(navigator_vr_);
  visitor->Trace(capabilities_);
  visitor->Trace(stage_parameters_);
  visitor->Trace(eye_parameters_left_);
  visitor->Trace(eye_parameters_right_);
  visitor->Trace(layer_);
  visitor->Trace(rendering_context_);
  visitor->Trace(scripted_animation_controller_);
  visitor->Trace(pending_present_resolvers_);
}

DEFINE_TRACE(VRDisplay) {
  EventTargetWithInlineData::Trace(visitor);
  ContextLifecycleObserver::Trace(visitor);
  visitor->Trace(navigator_vr_);
  visitor->Trace(capabilities_);
  visitor->Trace(stage_parameters_);
  visitor->Trace(eye_parameters_left_);
  visitor->Trace(eye_parameters_right_);
  visitor->Trace(layer_);
  visitor->Trace(rendering_context_);
  visitor->Trace(scripted_animation_controller_);
  visitor->Trace(pending_present_resolvers_);
}
