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=chromium:580959
LOG=n

Review-Url: https://codereview.chromium.org/1899813003
Cr-Commit-Position: refs/heads/master@{#36133}