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 SetupPendingTreeWithFixedTileSize(
      scoped_refptr<RasterSource> raster_source,
      const gfx::Size& tile_size,
      const Region& invalidation) {
    SetupPendingTreeInternal(raster_source, tile_size, invalidation);
  }

  void SetupPendingTreeWithFixedTileSize(
      scoped_refptr<RasterSource> raster_source,
      const gfx::Size& tile_size,
      const Region& invalidation) {
    SetupPendingTreeInternal(raster_source, tile_size, invalidation);
  }
