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}
PictureLayerImpl::Pair::Pair(PictureLayerImpl* active_layer,
                             PictureLayerImpl* pending_layer)
    : active(active_layer), pending(pending_layer) {
}

PictureLayerImpl::Pair::Pair(PictureLayerImpl* active_layer,
                             PictureLayerImpl* pending_layer)
    : active(active_layer), pending(pending_layer) {
}
