Revert "heap: Remove OSX restriction for maximum parallelism"

This reverts commit ad65e584.

Reason for revert: Regresses because of the same reasons for older OSX versions, see https://chromeperf.appspot.com/report?sid=4f570d131a97fdc45bc479f6e526d9caaa388eefca08242ef66cbf3b7eb850e4&start_rev=741028&end_rev=742546

Original change's description:
> heap: Remove OSX restriction for maximum parallelism
> 
> Change-Id: I582af41a8b10957b8fc7acb388871b1d51783a8c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062395
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#66317}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: I1812a39f2f70fd2b00d77b3cfd49edcd4e967d6d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2064212Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66334}

src/heap/concurrent-marking.cc

@@ -515,8 +515,17 @@ void ConcurrentMarking::ScheduleTasks() {
   if (total_task_count_ == 0) {
     static const int num_cores =
         V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
-    // Leave one core for the main thread and other components, respectively.
+#if defined(V8_OS_MACOSX)
+    // Mac OSX 10.11 and prior seems to have trouble when doing concurrent
+    // marking on competing hyper-threads (regresses Octane/Splay). As such,
+    // only use num_cores/2, leaving one of those for the main thread.
+    // TODO(ulan): Use all cores on Mac 10.12+.
+    total_task_count_ = Max(1, Min(kMaxTasks, (num_cores / 2) - 1));
+#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 - 2));
+#endif  // defined(OS_MACOSX)
     DCHECK_LE(total_task_count_, kMaxTasks);
     // One task is for the main thread.
     STATIC_ASSERT(kMaxTasks + 1 <= MarkingWorklist::kMaxNumTasks);