Port 61f5fbbb

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.

R=hpayer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=chromium:580959
LOG=N

Review-Url: https://codereview.chromium.org/1970633002
Cr-Commit-Position: refs/heads/master@{#36184}