- Separates measuring from wasm::SerializeNativeModule so caller
  can allocate or reserve the buffer memory. Call site thus avoid
  one unnecessary copy.

Bug: chromium:719007
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I50412c0c0279114dcdc1aead810ad39e7c4e2a6e
Reviewed-on: https://chromium-review.googlesource.com/1039183
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52965}