It was discovered that touching (to dirty pages) large regions of
memory can be very costly on Android (~60ms per 10MiB on Nexus 5).
We touch on IO thread, but still, it's better not to block it for
long periods.

With this CL we'll touch the ablation payload in chunks and we'll
schedule each touch individually to let other things to happen in
between.

BUG=710048

Review-Url: https://codereview.chromium.org/2896523004
Cr-Commit-Position: refs/heads/master@{#473459}