This reduces the critical section in {NativeModule::AddOwnedCode} to
not include the memcpy.
It also contains small drive-by fixes.

R=mstarzinger@chromium.org

Bug: v8:7921
Change-Id: Idbed5d1ab5627fe2ab3f60887ec2b31c5525e36c
Reviewed-on: https://chromium-review.googlesource.com/1127025
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54312}