Not doing so could result in this scenario:
- We ensure that the string table is large enough. It is.
- We compile code stubs, which triggers a GC.
- The GC clears string table entries.
- This increases the number of deleted entries in the table.
- When the deserializer hooks up internalized strings into the
  table, we ensure that the table is large enough every time.
- Due to changed number of deleted entries, the heuristic
  decides to expand the string table.
- Allocation during deserialization causes assertion to fail.

BUG=chromium:502085
LOG=N

Review URL: https://codereview.chromium.org/1197633002

Cr-Commit-Position: refs/heads/master@{#29134}