Update from chromium https://crrev.com/301315

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 43737f82778d942672d1d92f0303a4b802068b06..8bf92891a3ea967d897053f138d09030de808e62 100644
--- a/base/memory/discardable_memory_manager.h
+++ b/base/memory/discardable_memory_manager.h
@@ -31,6 +31,10 @@ class DiscardableMemoryManagerAllocation {
   // is acquired on the allocation.
   virtual void Purge() = 0;
 
+  // Check if allocated memory is still resident. It is illegal to call this
+  // while a lock is acquired on the allocation.
+  virtual bool IsMemoryResident() const = 0;
+
  protected:
   virtual ~DiscardableMemoryManagerAllocation() {}
 };
@@ -68,6 +72,9 @@ class BASE_EXPORT_PRIVATE DiscardableMemoryManager {
   void SetHardMemoryLimitExpirationTime(
       TimeDelta hard_memory_limit_expiration_time);
 
+  // This will make sure that all purged memory is released to the OS.
+  void ReleaseFreeMemory();
+
   // This will attempt to reduce memory footprint until within soft memory
   // limit. Returns true if there's no need to call this again until allocations
   // have been used.