Merge better captures the upcoming usecase in the wasm native heap,
where allocating/freeing is moving the accounting of memory from
a free list to an allocated list and vice-versa - making 'Release'
an odd API when allocating.

Bug: 
Change-Id: I9010959c91a1e8585eb06303ab06078132a03f60
Reviewed-on: https://chromium-review.googlesource.com/688004Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48211}