Add discardable memory emulation for non-android/mac platforms

base/memory/memory_pressure_listener.cc

Index: base/memory/memory_pressure_listener.cc
diff --git a/base/memory/memory_pressure_listener.cc b/base/memory/memory_pressure_listener.cc
index 911dbb38c89b86349f05aa392f4b4dfee56c3d88..76f83629c5cd5f14a7742510ce0ad321dec3d198 100644
--- a/base/memory/memory_pressure_listener.cc
+++ b/base/memory/memory_pressure_listener.cc
@@ -9,8 +9,29 @@
 
 namespace {
 
-base::LazyInstance<ObserverListThreadSafe<base::MemoryPressureListener> >::Leaky
-    g_observers = LAZY_INSTANCE_INITIALIZER;
+typedef
+    ObserverListThreadSafe<base::MemoryPressureListener>
+    PressureObserverList;
+
+// If the ObserverListThreadSafe<> were to be created by LazyInstance directly,
+// it would not have its refcount incremented and would be deleted as soon as
+// it tried to notify its observers. (As part of the notification process, it
+// creates closures holding scoped_refptrs to the global observer list. When
+// these closures are deleted, their scoped_refptrs get deleted and the global
+// list -- whose refcount just dropped to zero -- also gets deleted.) To avoid
+// this, we'll instead instantiate a wrapper holding a scoped_refptr to the
+// actual observer list.
+class ObserverListWrapper {
+ public:
+  ObserverListWrapper() : pressure_observers_(new PressureObserverList) {}
+  PressureObserverList& observers() { return *pressure_observers_; }
+
+ private:
+  scoped_refptr<PressureObserverList> pressure_observers_;
+};
+
+base::LazyInstance<ObserverListWrapper>::Leaky
+    g_observers_wrapper = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
@@ -19,11 +40,11 @@ namespace base {
 MemoryPressureListener::MemoryPressureListener(
     const MemoryPressureListener::MemoryPressureCallback& callback)
     : callback_(callback) {
-  g_observers.Get().AddObserver(this);
+  g_observers_wrapper.Get().observers().AddObserver(this);
 }
 
 MemoryPressureListener::~MemoryPressureListener() {
-  g_observers.Get().RemoveObserver(this);
+  g_observers_wrapper.Get().observers().RemoveObserver(this);
 }
 
 void MemoryPressureListener::Notify(MemoryPressureLevel memory_pressure_level) {
@@ -33,8 +54,8 @@ void MemoryPressureListener::Notify(MemoryPressureLevel memory_pressure_level) {
 // static
 void MemoryPressureListener::NotifyMemoryPressure(
     MemoryPressureLevel memory_pressure_level) {
-  g_observers.Get().Notify(&MemoryPressureListener::Notify,
-                           memory_pressure_level);
+  g_observers_wrapper.Get().observers().Notify(
+      &MemoryPressureListener::Notify, memory_pressure_level);
 }
 
 }  // namespace base