Re-land: base: Introduce an explicit call for reducing emulated discardable memory usage.

base/memory/discardable_memory_emulated.h

Index: base/memory/discardable_memory_emulated.h
diff --git a/base/memory/discardable_memory_emulated.h b/base/memory/discardable_memory_emulated.h
index 64e99511b7caad0ec835ec9cd245f262bf100b74..d928513527ea895f1e44ebd3a0415bfe446e3319 100644
--- a/base/memory/discardable_memory_emulated.h
+++ b/base/memory/discardable_memory_emulated.h
@@ -19,10 +19,13 @@ class DiscardableMemoryEmulated
   explicit DiscardableMemoryEmulated(size_t bytes);
   virtual ~DiscardableMemoryEmulated();
 
-  static void RegisterMemoryPressureListeners();
-  static void UnregisterMemoryPressureListeners();
   static bool ReduceMemoryUsage();
 
+  // TODO(reveman): Remove this as it is breaking the discardable memory design
+  // principle that implementations should not rely on information this is
+  // unavailable in kernel space. crbug.com/400423
+  BASE_EXPORT static void ReduceMemoryUsageUntilWithinLimit(size_t bytes);
+
   static void PurgeForTesting();
 
   bool Initialize();