The {code_table_} in {NativeModule} is protected by the
{allocation_mutex_}. The {code} and {code_table} accessors did not
acquire this lock though.
This CL removes the unsafe {code_table} accessor, renames {code} to
{GetCode} and protects it by a lock.

R=mstarzinger@chromium.org

Bug: v8:9112
Change-Id: Id2df68460b4c10291a49b4016b9574e02744e8b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1561315Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60779}