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}
  static void VerifyAllTilesExistAndHavePile(
      const PictureLayerTiling* tiling,
      PicturePileImpl* pile) {
    for (PictureLayerTiling::CoverageIterator iter(
             tiling,
             tiling->contents_scale(),
             gfx::Rect(tiling->tiling_size()));
         iter;
         ++iter) {
      EXPECT_TRUE(*iter);
      EXPECT_EQ(pile, iter->raster_source());
    }
  }

  static void VerifyAllTilesExistAndHavePile(
      const PictureLayerTiling* tiling,
      PicturePileImpl* pile) {
    for (PictureLayerTiling::CoverageIterator iter(
             tiling,
             tiling->contents_scale(),
             gfx::Rect(tiling->tiling_size()));
         iter;
         ++iter) {
      EXPECT_TRUE(*iter);
      EXPECT_EQ(pile, iter->raster_source());
    }
  }
