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

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 0f85c1fe903d612ecf055043c1a0d84d362ddbf7..d0eaca1a66219eafee72db30d3e91170e4055e98 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -340,6 +340,15 @@ bool DiscardableSharedMemory::IsMemoryResident() const {
          !result.GetTimestamp().is_null();
 }
 
+bool DiscardableSharedMemory::IsMemoryLocked() const {
+  DCHECK(shared_memory_.memory());
+
+  SharedState result(subtle::NoBarrier_Load(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i));
+
+  return result.GetLockState() == SharedState::LOCKED;
+}
+
 void DiscardableSharedMemory::Close() {
   shared_memory_.Close();
 }