Background to this is that blink needs to be able to pass different internal
fields deserialization callbacks for individual to-be-deserialized contexts.

R=jochen@chromium.org, peria@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2619203002
Cr-Commit-Position: refs/heads/master@{#42132}