[heap] Remove Mutex in PagedSpace::Expand

Locking the space mutex isn't required in PagedSpace::Expand.

Bug: v8:10315
Change-Id: I6fac74e3c4fc6f4f2228af37729839b6ff23bc64
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207183
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67889}

src/heap/paged-spaces.cc

@@ -318,11 +318,6 @@ void PagedSpace::ShrinkImmortalImmovablePages() {
 }
 
 Page* PagedSpace::Expand() {
-  // TODO(ulan): Remove the mutex as it seems redundant:
-  // Always lock against the main space as we can only adjust capacity and
-  // pages concurrently for the main paged space.
-  base::MutexGuard guard(heap()->paged_space(identity())->mutex());
-
   Page* page =
       heap()->memory_allocator()->AllocatePage(AreaSize(), this, executable());
   if (page == nullptr) return nullptr;