- Rename WasmCode::owner() to WasmCode::native_module() and
- Make {shared} field of WasmCompiledModule no-longer const, since
  it had a setter masquerading under the
  {OnWasmModuleDeserialization()}.
- Refactor and simplify the flow of "owner" in module-compiler.cc

R=mstarzinger@chromium.org

Change-Id: If9ee371124678fbbc845fc4e93279bf14f8f7ce8
Reviewed-on: https://chromium-review.googlesource.com/964263Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51959}