Use MADV_FREE_REUSABLE instead of MADV_FREE in partition alloc for macOS.

base/allocator/partition_allocator/page_allocator.cc

Index: base/allocator/partition_allocator/page_allocator.cc
diff --git a/base/allocator/partition_allocator/page_allocator.cc b/base/allocator/partition_allocator/page_allocator.cc
index 606155fce4a3075c753a8634560a4f69c247509c..bb737b3341986d73739a2f5bed84d0c33052b1ce 100644
--- a/base/allocator/partition_allocator/page_allocator.cc
+++ b/base/allocator/partition_allocator/page_allocator.cc
@@ -216,7 +216,14 @@ bool SetSystemPagesAccessible(void* address, size_t length) {
 void DecommitSystemPages(void* address, size_t length) {
   DCHECK(!(length & kSystemPageOffsetMask));
 #if defined(OS_POSIX)
+#if defined(OS_MACOSX)
+  // On macOS, 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.
+  int ret = madvise(address, length, MADV_FREE_REUSABLE);
+#else
   int ret = madvise(address, length, MADV_FREE);
+#endif
   if (ret != 0 && errno == EINVAL) {
     // MADV_FREE only works on Linux 4.5+ . If request failed,
     // retry with older MADV_DONTNEED . Note that MADV_FREE