macOS: Use MADV_FREE_REUSABLE for discardable shared memory.

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index c04e5c41c58d0b7945afcafb25e7d5cd3b5aee05..99d2991308a3cd503543f5c6da2cbcc6ef1765e1 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -344,6 +344,11 @@ bool DiscardableSharedMemory::Purge(Time current_time) {
 // provide MADV_FREE which has the same result but memory is purged lazily.
 #if defined(OS_LINUX) || defined(OS_ANDROID)
 #define MADV_PURGE_ARGUMENT MADV_REMOVE
+#elif defined(OS_MACOSX)
+// MADV_FREE_REUSABLE is similar 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.
+#define MADV_PURGE_ARGUMENT MADV_FREE_REUSABLE
 #else
 #define MADV_PURGE_ARGUMENT MADV_FREE
 #endif