This CL simply moves the implementation classes for WASM native
module serialization into the CC file and simplifies them a bit.

R=mstarzinger@chromium.org

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Id560d2c35eb12bdd461f5d80cb5211a2f4f51684
Reviewed-on: https://chromium-review.googlesource.com/856677Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50434}