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}
bool VRDisplay::getFrameData(VRFrameData* frame_data) {
  if (!IsPresentationFocused() || !frame_pose_ || display_blurred_)
    return false;

  if (!frame_data)
    return false;

  if (depth_near_ == depth_far_)
    return false;

  return frame_data->Update(frame_pose_, eye_parameters_left_,
                            eye_parameters_right_, depth_near_, depth_far_);
}

bool VRDisplay::getFrameData(VRFrameData* frame_data) {
  if (!IsPresentationFocused() || !frame_pose_ || display_blurred_)
    return false;

  if (!frame_data)
    return false;

  if (depth_near_ == depth_far_)
    return false;

  return frame_data->Update(frame_pose_, eye_parameters_left_,
                            eye_parameters_right_, depth_near_, depth_far_);
}
