The alternative of passing an object template at context creation is
unfeasible because we need a context to instantiate the template.
At the time we create the context from snapshot or bootstrap from
scratch, we would already need that template instance, leading to a
chicken-and-egg problem.

This is an alternative that is simpler and less intrusive.

R=domenic@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/1182443002

Cr-Commit-Position: refs/heads/master@{#28938}