Added memory pressure monitor for linux.

base/memory/memory_pressure_monitor.cc

Index: base/memory/memory_pressure_monitor.cc
diff --git a/base/memory/memory_pressure_monitor.cc b/base/memory/memory_pressure_monitor.cc
index 00633f1dd61981af41c7738d9c182d97367b9aef..7f27cf69e0275d8503c6181c34f9c4461ad7f4ff 100644
--- a/base/memory/memory_pressure_monitor.cc
+++ b/base/memory/memory_pressure_monitor.cc
@@ -7,21 +7,9 @@
 #include "base/logging.h"
 
 namespace base {
-namespace {
 
-MemoryPressureMonitor* g_monitor = nullptr;
-
-}  // namespace
-
-MemoryPressureMonitor::MemoryPressureMonitor() {
-  DCHECK(!g_monitor);
-  g_monitor = this;
-}
-
-MemoryPressureMonitor::~MemoryPressureMonitor() {
-  DCHECK(g_monitor);
-  g_monitor = nullptr;
-}
+// static
+MemoryPressureMonitor* MemoryPressureMonitor::g_monitor = nullptr;
 
 // static
 MemoryPressureMonitor* MemoryPressureMonitor::Get() {