Use conservative hash table capacity growth during entire snapshotting.

We want to use the conservative growth strategy during
- isolate initialization
- bootstrapping a context

But not when
- not creating a snapshot
- running additional code for custom snapshot.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1155823003

Cr-Commit-Position: refs/heads/master@{#28586}