Added memory pressure monitor for linux.

base/memory/memory_pressure_monitor_win.cc

Index: base/memory/memory_pressure_monitor_win.cc
diff --git a/base/memory/memory_pressure_monitor_win.cc b/base/memory/memory_pressure_monitor_win.cc
index 4349d035806c3efe6689c0846b7d787a11e467ce..9e50444a62f52bc64f68e83a0befe12ccbd3bbf3 100644
--- a/base/memory/memory_pressure_monitor_win.cc
+++ b/base/memory/memory_pressure_monitor_win.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_win.h"
+#include "base/memory/memory_pressure_monitor.h"
 
 #include <windows.h>
 
@@ -12,7 +12,6 @@
 #include "base/time/time.h"
 
 namespace base {
-namespace win {
 
 namespace {
 
@@ -95,12 +94,18 @@ MemoryPressureMonitor::MemoryPressureMonitor(int moderate_threshold_mb,
           MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE),
       moderate_pressure_repeat_count_(0),
       weak_ptr_factory_(this) {
+  DCHECK(!g_monitor);
+  g_monitor = this;
+
   DCHECK_GE(moderate_threshold_mb_, critical_threshold_mb_);
   DCHECK_LE(0, critical_threshold_mb_);
   StartObserving();
 }
 
 MemoryPressureMonitor::~MemoryPressureMonitor() {
+  DCHECK(g_monitor);
+  g_monitor = nullptr;
+
   StopObserving();
 }
 
@@ -250,5 +255,4 @@ bool MemoryPressureMonitor::GetSystemMemoryStatus(
   return true;
 }
 
-}  // namespace win
 }  // namespace base