This part of the snapshot API should not be in use yet, so we can still
change this. The motivation for this change is:
- Use MaybeHandle where reasonable.
- Remove ambiguity: when we use index to create context from snapshot,
  we should not have a silent fallback if snapshot is not available.
- Symmetry: rename to Context::FromSnapshot to mirror templates.

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

Review-Url: https://codereview.chromium.org/2100073002
Cr-Commit-Position: refs/heads/master@{#37334}