This is the 3rd step in a series of CLs to move the SharedFunctionInfo
class to kNeverSerialized and make it concurrently accessible from
the background thread. This CL:
* Adds synchronization to PrepareFunctionForDebugExecution
* Adds tests that mess with SharedFunctionInfo while it is accessed
  by another thread.

Bug: v8:7790
Change-Id: I2200fc7b6e977cda4e1003cb83d6ff49b1f1e337
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2523318Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71495}