c:: Give the raster source to the PictureLayerTilings.

We were passing around just the layer_bounds, and passing the raster
source through the tilings to the tiles. Instead, let the tiling save
the raster source and then it can skip creating unrasterable tiles
itself making the PictureLayerImpl::CreateTile no longer able to return
a nullptr.

R=enne, vmpstr
BUG=387116

Review URL: https://codereview.chromium.org/820703002

Cr-Commit-Position: refs/heads/master@{#309251}
14 files changed