[heap] Properly decrement amount of externally allocated memory
[heap] Properly decrement amount of externally allocated memory

TBR=mstarzinger@chromium.org
BUG=chromium:526244
LOG=N

Review URL: https://codereview.chromium.org/1329493002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30487}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh