This CL removes dependence on the ForegroundTaskRunner in the shared
isolate because the shared isolate doesn't have one associated with it.

There are 3 places:

1. The memory reducer is no longer allocated on the shared isolate.

2. The WasmEngine is no longer initialized for the shared isolate, which
   never executes user code.

3. Ability to post non-nestable tasks is no longer required on the
   shared isolate. This is required for FinalizationRegistry, and
   the shared isolate never executes user code.

Bug: v8:11708
Change-Id: I4b0cf2c8dc8686ccc7b7d24e6c9e12eb4b9d03d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3864275
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82876}