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 SetupDefaultTreesWithInvalidation(const gfx::Size& layer_bounds,
                                         const Region& invalidation) {
    gfx::Size tile_size(100, 100);

    scoped_refptr<FakePicturePileImpl> pending_pile =
        FakePicturePileImpl::CreateFilledPile(tile_size, layer_bounds);
    scoped_refptr<FakePicturePileImpl> active_pile =
        FakePicturePileImpl::CreateFilledPile(tile_size, layer_bounds);

    SetupTreesWithInvalidation(pending_pile, active_pile, invalidation);
  }

  void SetupDefaultTreesWithInvalidation(const gfx::Size& layer_bounds,
                                         const Region& invalidation) {
    gfx::Size tile_size(100, 100);

    scoped_refptr<FakePicturePileImpl> pending_pile =
        FakePicturePileImpl::CreateFilledPile(tile_size, layer_bounds);
    scoped_refptr<FakePicturePileImpl> active_pile =
        FakePicturePileImpl::CreateFilledPile(tile_size, layer_bounds);

    SetupTreesWithInvalidation(pending_pile, active_pile, invalidation);
  }
