Most code protected by compilation_scope_mutex_ is already either thread
safe, or could run in parallel. Removing lock reduces contention.
Note that weak_ptr::lock is atomic and thus still prevents deletion
of NativeModule&CompilationStateImpl for the scope of
BackgroundCompileScope.
Related changes:
- BackgroundCompileToken is deleted and publish_queue is moved to
  CompilationStateImpl.
- Some of the (non thread-safe) logic in publish_results is moved into
  PublishCompilationResults so that it is serialized to 1 thread
  running publisher.
- cancellation is handled by an atomic bool and is no longer
  synchronized. This means that compilation may be cancelled while
  a worker thread is still running. That thread would only
  stop once it reaches a new BackgroundCompileScope.

Change-Id: I9651e924857c583d1a0fe5b9ffa99bfd01a8bda4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2442192Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70574}