Fix OS_MACOSX usages

Two of them were in comments; updated them to V8_OS_MACOSX.

Two of them were incorrectly in #if statements. Updated them to
V8_OS_MACOSX.

Bug: chromium:823915, chromium:1105907
Change-Id: Ibfc0f8936dbc8cbf3b05a674e882bbc480d0b4c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2331736Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69178}

src/base/platform/platform-posix.cc

@@ -419,7 +419,7 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
 // The cost is a syscall that effectively no-ops.
 // TODO(erikchen): Fix this to only call MADV_FREE_REUSE when necessary.
 // https://crbug.com/823915
-#if defined(OS_MACOSX)
+#if defined(V8_OS_MACOSX)
   if (access != OS::MemoryPermission::kNoAccess)
     madvise(address, size, MADV_FREE_REUSE);
 #endif
@@ -430,7 +430,7 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
 bool OS::DiscardSystemPages(void* address, size_t size) {
   DCHECK_EQ(0, reinterpret_cast<uintptr_t>(address) % CommitPageSize());
   DCHECK_EQ(0, size % CommitPageSize());
-#if defined(OS_MACOSX)
+#if defined(V8_OS_MACOSX)
   // On OSX, MADV_FREE_REUSABLE has comparable behavior to MADV_FREE, but also
   // marks the pages with the reusable bit, which allows both Activity Monitor
   // and memory-infra to correctly track the pages.

src/heap/concurrent-marking.cc

@@ -519,11 +519,11 @@ void ConcurrentMarking::ScheduleTasks() {
     // 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)
+#else   // defined(V8_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)
+#endif  // defined(V8_OS_MACOSX)
     if (FLAG_gc_experiment_reduce_concurrent_marking_tasks) {
       // Use at most half of the cores in the experiment.
       total_task_count_ = Max(1, Min(kMaxTasks, (num_cores / 2) - 1));