Re-land: base: Fix registering of memory pressure listeners used when emulating discardable memory.

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index 7c7785531ed4b365a1725c3594a9682621428712..c39d0db1673714a774a2fbea4a4ebd9a01371c3c 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -63,6 +63,14 @@ class BASE_EXPORT DiscardableMemory {
  public:
   virtual ~DiscardableMemory() {}
 
+  // Call this on a thread with a MessageLoop current to allow discardable
+  // memory implementations to respond to memory pressure signals.
+  static void RegisterMemoryPressureListeners();
+
+  // Call this to prevent discardable memory implementations from responding
+  // to memory pressure signals.
+  static void UnregisterMemoryPressureListeners();
+
   // Gets the discardable memory type with a given name.
   static DiscardableMemoryType GetNamedType(const std::string& name);