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}
PictureLayerImpl::~PictureLayerImpl() {
  if (twin_layer_)
    twin_layer_->twin_layer_ = nullptr;
  layer_tree_impl()->UnregisterPictureLayerImpl(this);
}

PictureLayerImpl::~PictureLayerImpl() {
  if (twin_layer_)
    twin_layer_->twin_layer_ = nullptr;
  layer_tree_impl()->UnregisterPictureLayerImpl(this);
}
