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}
scoped_ptr<TilingSetEvictionQueue> PictureLayerImpl::CreateEvictionQueue(
    TreePriority tree_priority) {
  if (!tilings_)
    return make_scoped_ptr(new TilingSetEvictionQueue());
  bool skip_shared_out_of_order_tiles =
      GetPendingOrActiveTwinLayer() != nullptr;
  return make_scoped_ptr(new TilingSetEvictionQueue(
      tilings_.get(), tree_priority, skip_shared_out_of_order_tiles));
}

scoped_ptr<TilingSetEvictionQueue> PictureLayerImpl::CreateEvictionQueue(
    TreePriority tree_priority) {
  if (!tilings_)
    return make_scoped_ptr(new TilingSetEvictionQueue());
  bool skip_shared_out_of_order_tiles =
      GetPendingOrActiveTwinLayer() != nullptr;
  return make_scoped_ptr(new TilingSetEvictionQueue(
      tilings_.get(), tree_priority, skip_shared_out_of_order_tiles));
}
