For wasm memory buffers, we normally require the memory to be allocated
via WebAssembly.Memory, which will set a is_wasm_memory flag and
register the memory with the wasm allocation tracker. This CL weakens
that requirement in a DCHECK to allow for running experimental threaded
applications even though the is_wasm_memory flag is not currently
propagated via postMessage.

Bug: chromium:836800
Change-Id: I4613b8651423307ce4cd466c0df28fc43244ec4f
Reviewed-on: https://chromium-review.googlesource.com/1027813
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52801}