PagedSpace::SizeOfObjects() then returns exactly the same value as
PagedSpace::Size(). SizeOfObjects() used to deduct the current LAB,
however this is now more difficult with local heaps. Accessing the
main thread LAB from concurrent threads causes a data race. Also
LocalHeaps have their own LAB, which should be deducted as well to be
uniform with the main thread. However this would be tricky and expensive.
The simpler solution is to do not deduct the main thread LAB anymore.

Bug: v8:10315
Change-Id: I3c47e1a65caca9395737251aa694b295e78c7fb5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2336090
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69245}