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}
bool PictureLayerImpl::RequiresHighResToDraw() const {
  return layer_tree_impl()->RequiresHighResToDraw();
}

bool PictureLayerImpl::RequiresHighResToDraw() const {
  return layer_tree_impl()->RequiresHighResToDraw();
}
