When a field type is a map, it is wrapped in a weak cell upon storing to the descriptor array.

Map::GetFieldType(i) does the unwrapping.

BUG=v8:3877
LOG=N
TEST=cctest/test-heap/Regress3877

Review URL: https://codereview.chromium.org/955063002

Cr-Commit-Position: refs/heads/master@{#26879}