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}
void PictureLayerImpl::SetNearestNeighbor(bool nearest_neighbor) {
  if (nearest_neighbor_ == nearest_neighbor)
    return;

  nearest_neighbor_ = nearest_neighbor;
  NoteLayerPropertyChanged();
}

void PictureLayerImpl::SetNearestNeighbor(bool nearest_neighbor) {
  if (nearest_neighbor_ == nearest_neighbor)
    return;

  nearest_neighbor_ = nearest_neighbor;
  NoteLayerPropertyChanged();
}
