[heap] Use less tasks for concurrrent marking

Use 1 less task for concurrent marking to allow Oilpan's concurrent
marking a chance to run as well.

Change-Id: I236f78925324dc4d21a38233d75dc101cc477930
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062389
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66311}

src/heap/concurrent-marking.cc

@@ -524,7 +524,7 @@ void ConcurrentMarking::ScheduleTasks() {
 #else   // defined(OS_MACOSX)
     // On other platforms use all logical cores, leaving one for the main
     // thread.
-    total_task_count_ = Max(1, Min(kMaxTasks, num_cores - 1));
+    total_task_count_ = Max(1, Min(kMaxTasks, num_cores - 2));
 #endif  // defined(OS_MACOSX)
     DCHECK_LE(total_task_count_, kMaxTasks);
     // One task is for the main thread.