Reland of [tracing] Display the locked size of discardable memory segment.

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index 74bbe8e95f427ba347d696e6c6e9c408e166e167..5dca884ca82e2d6665930905c92e990b8a5515c2 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -107,6 +107,9 @@ class BASE_EXPORT DiscardableSharedMemory {
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;
 
+  // Returns true if memory is locked.
+  bool IsMemoryLocked() const;
+
   // Closes the open discardable memory segment.
   // It is safe to call Close repeatedly.
   void Close();