Added memory pressure monitor for linux.

base/memory/memory_pressure_monitor_mac.cc

Index: base/memory/memory_pressure_monitor_mac.cc
diff --git a/base/memory/memory_pressure_monitor_mac.cc b/base/memory/memory_pressure_monitor_mac.cc
index f394935acb99ffe2738816dd722c415ab2e875d5..17cd455e77d03b37f33a3d836df10170cfe67963 100644
--- a/base/memory/memory_pressure_monitor_mac.cc
+++ b/base/memory/memory_pressure_monitor_mac.cc
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/memory/memory_pressure_monitor_mac.h"
+#include "base/memory/memory_pressure_monitor.h"
 
 #include <dlfcn.h>
 #include <sys/sysctl.h>
@@ -10,7 +10,6 @@
 #include "base/mac/mac_util.h"
 
 namespace base {
-namespace mac {
 
 MemoryPressureListener::MemoryPressureLevel
 MemoryPressureMonitor::MemoryPressureLevelForMacMemoryPressure(
@@ -36,6 +35,9 @@ void MemoryPressureMonitor::NotifyMemoryPressureChanged(
 
 MemoryPressureMonitor::MemoryPressureMonitor()
   : memory_level_event_source_(nullptr) {
+  DCHECK(!g_monitor);
+  g_monitor = this;
+
   // _dispatch_source_type_memorypressure is not available prior to 10.9.
   dispatch_source_type_t dispatch_source_memorypressure =
       static_cast<dispatch_source_type_t>
@@ -60,6 +62,8 @@ MemoryPressureMonitor::MemoryPressureMonitor()
 }
 
 MemoryPressureMonitor::~MemoryPressureMonitor() {
+  DCHECK(g_monitor);
+  g_monitor = nullptr;
 }
 
 MemoryPressureListener::MemoryPressureLevel
@@ -74,5 +78,4 @@ MemoryPressureMonitor::GetCurrentPressureLevel() const {
   return MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
 }
 
-}  // namespace mac
 }  // namespace base