port 61f5fbbb (r36133)

  original commit message:
  The new allocation folding implementation avoids fragmentation between folded allocation. As a consequence, our heap will always be iterable i.e. we do not have to perform a garbage collection
  before iterating the heap.

BUG=

Review-Url: https://codereview.chromium.org/1969553003
Cr-Commit-Position: refs/heads/master@{#36158}