[heap] Fix MemoryChunk::kHeaderSize computation and add some assertions.
[heap] Fix MemoryChunk::kHeaderSize computation and add some assertions.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1302423007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30630}

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