Added memory pressure monitor for linux.

base/memory/memory_pressure_monitor_chromeos.cc

Index: base/memory/memory_pressure_monitor_chromeos.cc
diff --git a/base/memory/memory_pressure_monitor_chromeos.cc b/base/memory/memory_pressure_monitor_chromeos.cc
index 640e4633fd51c7cb9d3e820af18c62a17ea14f0b..ccf7957d19849baca65b764de4f93132b0df951a 100644
--- a/base/memory/memory_pressure_monitor_chromeos.cc
+++ b/base/memory/memory_pressure_monitor_chromeos.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_chromeos.h"
+#include "base/memory/memory_pressure_monitor.h"
 
 #include <fcntl.h>
 #include <sys/select.h>
@@ -15,7 +15,6 @@
 #include "base/time/time.h"
 
 namespace base {
-namespace chromeos {
 
 namespace {
 
@@ -113,11 +112,17 @@ MemoryPressureMonitor::MemoryPressureMonitor(
           GetCriticalMemoryThresholdInPercent(thresholds)),
       low_mem_file_(HANDLE_EINTR(::open(kLowMemFile, O_RDONLY))),
       weak_ptr_factory_(this) {
+  DCHECK(!g_monitor);
+  g_monitor = this;
+
   StartObserving();
   LOG_IF(ERROR, !low_mem_file_.is_valid()) << "Cannot open kernel listener";
 }
 
 MemoryPressureMonitor::~MemoryPressureMonitor() {
+  DCHECK(g_monitor);
+  g_monitor = nullptr;
+
   StopObserving();
 }
 
@@ -132,12 +137,6 @@ MemoryPressureMonitor::GetCurrentPressureLevel() const {
   return current_memory_pressure_level_;
 }
 
-// static
-MemoryPressureMonitor* MemoryPressureMonitor::Get() {
-  return static_cast<MemoryPressureMonitor*>(
-      base::MemoryPressureMonitor::Get());
-}
-
 void MemoryPressureMonitor::StartObserving() {
   timer_.Start(FROM_HERE,
                TimeDelta::FromMilliseconds(kMemoryPressureIntervalMs),
@@ -268,5 +267,4 @@ int MemoryPressureMonitor::GetUsedMemoryInPercent() {
   return percentage;
 }
 
-}  // namespace chromeos
 }  // namespace base