Most maps have a small code cache (often only one entry), so this patch
optimizes memory consumption of such cases by using plain FixedArrays,
only switching to CodeCacheHashTables when the number of cached entries
gets so large that linear-scan lookups get too slow.

On loading inbox.google.com, this gets the aggregate size of all maps'
code caches (there are about 13,600 of them) from 4300 KB to 970 KB.

Review-Url: https://codereview.chromium.org/2021373002
Cr-Commit-Position: refs/heads/master@{#36681}