This patch adds HeapObject::set_map_after_allocation method that
initializes the map of the object without object layout checks.

All other map setters now check that transitions unsafe for
concurrent marking properly notify the GC.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2885883004
Cr-Commit-Position: refs/heads/master@{#45403}