So far we only snapshot the payload data for the deserializer, but
not the data for heap reservation or the code stub keys.

At the same time this change turns some CHECKs that fail in the wild
to DCHECKs. We can try turning them back to CHECKs some other time.

TBR=ahaas@chromium.org
BUG=chromium:636876

Review-Url: https://codereview.chromium.org/2237883002
Cr-Commit-Position: refs/heads/master@{#38594}