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<LayerImpl> PictureLayer::CreateLayerImpl(LayerTreeImpl* tree_impl) {
  return PictureLayerImpl::Create(tree_impl, id(), is_mask_);
}

scoped_ptr<LayerImpl> PictureLayer::CreateLayerImpl(LayerTreeImpl* tree_impl) {
  return PictureLayerImpl::Create(tree_impl, id(), is_mask_);
}
