This moves the Wasm-specific metadata from being fields on the
ArrayBuffer into a table managed by WasmMemoryTracker.

Bug: chromium:776273
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Id8b050bfdfe0fbe9436fb055e92c08d503d3c2ba
Reviewed-on: https://chromium-review.googlesource.com/850550
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52080}