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 AssertAllTilesRequired(PictureLayerTiling* tiling) {
    std::vector<Tile*> tiles = tiling->AllTilesForTesting();
    for (size_t i = 0; i < tiles.size(); ++i)
      EXPECT_TRUE(tiles[i]->required_for_activation()) << "i: " << i;
    EXPECT_GT(tiles.size(), 0u);
  }

  void AssertAllTilesRequired(PictureLayerTiling* tiling) {
    std::vector<Tile*> tiles = tiling->AllTilesForTesting();
    for (size_t i = 0; i < tiles.size(); ++i)
      EXPECT_TRUE(tiles[i]->required_for_activation()) << "i: " << i;
    EXPECT_GT(tiles.size(), 0u);
  }
