This CL removes GCTracer::AssertMainThread and adds the more general
methods Heap::IsMainThread and Heap::IsSharedMainThread, to be used
in DCHECKs and elsewhere. It also introduces some const qualifiers.

Bug: v8:12425
Change-Id: Ibdec39ce77be704598ca0c8b440005dc27bd6997
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3650600Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80586}