content: Release all free discardable memory when idle.

content/child/child_discardable_shared_memory_manager.h

Index: content/child/child_discardable_shared_memory_manager.h
diff --git a/content/child/child_discardable_shared_memory_manager.h b/content/child/child_discardable_shared_memory_manager.h
index de4e7c79ea40c0a1fbba608bb3356e0d1541121e..27c8b83c9feea3074b5ee07652007b2c43c2b3df 100644
--- a/content/child/child_discardable_shared_memory_manager.h
+++ b/content/child/child_discardable_shared_memory_manager.h
@@ -36,12 +36,12 @@ class CONTENT_EXPORT ChildDiscardableSharedMemoryManager
  private:
   scoped_ptr<base::DiscardableSharedMemory>
   AllocateLockedDiscardableSharedMemory(size_t size);
-  void BytesAllocatedChanged(size_t new_bytes_allocated) const;
+  void MemoryUsageChanged(size_t new_bytes_allocated,
+                          size_t new_bytes_free) const;
 
   mutable base::Lock lock_;
   DiscardableSharedMemoryHeap heap_;
   scoped_refptr<ThreadSafeSender> sender_;
-  size_t bytes_allocated_;
 
   DISALLOW_COPY_AND_ASSIGN(ChildDiscardableSharedMemoryManager);
 };