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}
  int IterateAndCount(DrawQuad* quad) {
    num_resources_ = 0;
    quad->IterateResources(base::Bind(
        &DrawQuadIteratorTest::IncrementResourceId, base::Unretained(this)));
    return num_resources_;
  }

  int IterateAndCount(DrawQuad* quad) {
    num_resources_ = 0;
    quad->IterateResources(base::Bind(
        &DrawQuadIteratorTest::IncrementResourceId, base::Unretained(this)));
    return num_resources_;
  }
