WebAssembly creates ArrayBuffers with large allocations where only a small
amount is committed. The uncommitted address space should not be counted as used
memory. Doing so can lead to the GC spending unnecessary time collecting memory
when there is not really pressure.

Bug: 
Change-Id: Ife7b84e9858e87faabc360a61f887b2fda6d99db
Reviewed-on: https://chromium-review.googlesource.com/710227Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48462}