[heap] Make PagedSpace::Available lock mutex

Isolate::GetHeapStatistics uses PagedSpace::Available, which races
with allocating background threads.

Bug: v8:10315
Change-Id: I6e0dc37d90e0c7a3e3dd2b8bdb77f2ea82372c13
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2349294Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69337}

src/heap/paged-spaces.cc

@@ -433,6 +433,17 @@ void PagedSpace::MakeLinearAllocationAreaIterable() {
   }
 }
 
+size_t PagedSpace::Available() {
+  base::Optional<base::MutexGuard> optional_mutex;
+
+  if (FLAG_concurrent_allocation && identity() == OLD_SPACE &&
+      !is_local_space()) {
+    optional_mutex.emplace(&allocation_mutex_);
+  }
+
+  return free_list_->Available();
+}
+
 void PagedSpace::FreeLinearAllocationArea() {
   // Mark the old linear allocation area with a free space map so it can be
   // skipped when scanning the heap.

src/heap/paged-spaces.h

@@ -112,7 +112,7 @@ class V8_EXPORT_PRIVATE PagedSpace
   // The bytes in the linear allocation area are not included in this total
   // because updating the stats would slow down allocation.  New pages are
   // immediately added to the free list so they show up here.
-  size_t Available() override { return free_list_->Available(); }
+  size_t Available() override;
 
   // Allocated bytes in this space.  Garbage bytes that were not found due to
   // concurrent sweeping are counted as being allocated!  The bytes in the