The reloading was needed when GC would compact the Heap::retained_maps
array. But that's no longer true; the compaction is done in
Heap::AddRetainedMap, outside GC. So it's not possible that the length would
change because of an allocation.

(Pre-cleanup for in-place weak ref work.)

BUG=v8:7308

Change-Id: I18554353014865992f9151002cc4097fb986faf1
Reviewed-on: https://chromium-review.googlesource.com/1002775Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52506}