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

base/memory/discardable_memory_manager.h

Index: base/memory/discardable_memory_manager.h
diff --git a/base/memory/discardable_memory_manager.h b/base/memory/discardable_memory_manager.h
index a61f141c43034ecb5f1cfcec5033b8d431d884d8..8958e9d13038c08c0c4f5c01c901c3b17c4d1a73 100644
--- a/base/memory/discardable_memory_manager.h
+++ b/base/memory/discardable_memory_manager.h
@@ -8,7 +8,6 @@
 #include "base/base_export.h"
 #include "base/containers/hash_tables.h"
 #include "base/containers/mru_cache.h"
-#include "base/memory/memory_pressure_listener.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
@@ -74,13 +73,6 @@ class BASE_EXPORT_PRIVATE DiscardableMemoryManager {
                            TimeDelta hard_memory_limit_expiration_time);
   virtual ~DiscardableMemoryManager();
 
-  // Call this to register memory pressure listener. Must be called on a thread
-  // with a MessageLoop current.
-  void RegisterMemoryPressureListener();
-
-  // Call this to unregister memory pressure listener.
-  void UnregisterMemoryPressureListener();
-
   // The maximum number of bytes of memory that may be allocated before we force
   // a purge.
   void SetMemoryLimit(size_t bytes);
@@ -101,6 +93,10 @@ class BASE_EXPORT_PRIVATE DiscardableMemoryManager {
   // have been used.
   bool ReduceMemoryUsage();
 
+  // This can be called to attempt to reduce memory footprint until within
+  // limit for bytes to keep under moderate pressure.
+  void ReduceMemoryUsageUntilWithinModeratePressureLimit();
+
   // Adds the given allocation to the manager's collection.
   void Register(Allocation* allocation, size_t bytes);
 
@@ -141,10 +137,6 @@ class BASE_EXPORT_PRIVATE DiscardableMemoryManager {
   };
   typedef HashingMRUCache<Allocation*, AllocationInfo> AllocationMap;
 
-  // This can be called as a hint that the system is under memory pressure.
-  void OnMemoryPressure(
-      MemoryPressureListener::MemoryPressureLevel pressure_level);
-
   // Purges memory until usage is less or equal to
   // |bytes_to_keep_under_moderate_pressure_|.
   void PurgeUntilWithinBytesToKeepUnderModeratePressure();
@@ -189,10 +181,6 @@ class BASE_EXPORT_PRIVATE DiscardableMemoryManager {
   // this limit.
   size_t bytes_to_keep_under_moderate_pressure_;
 
-  // Allows us to be respond when the system reports that it is under memory
-  // pressure.
-  scoped_ptr<MemoryPressureListener> memory_pressure_listener_;
-
   // Amount of time it takes for an allocation to become affected by
   // |soft_memory_limit_|.
   TimeDelta hard_memory_limit_expiration_time_;