cc: Make the PictureLayerImpl raster source null until commit.

No need to make a raster source that is never used.

R=enne, vmpstr
BUG=387116

Review URL: https://codereview.chromium.org/809433003

Cr-Commit-Position: refs/heads/master@{#308466}
const PictureLayerTiling* PictureLayerImpl::GetPendingOrActiveTwinTiling(
    const PictureLayerTiling* tiling) const {
  PictureLayerImpl* twin_layer = GetPendingOrActiveTwinLayer();
  if (!twin_layer)
    return nullptr;
  return twin_layer->tilings_->FindTilingWithScale(tiling->contents_scale());
}

const PictureLayerTiling* PictureLayerImpl::GetPendingOrActiveTwinTiling(
    const PictureLayerTiling* tiling) const {
  PictureLayerImpl* twin_layer = GetPendingOrActiveTwinLayer();
  if (!twin_layer)
    return nullptr;
  return twin_layer->tilings_->FindTilingWithScale(tiling->contents_scale());
}
