[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..25b29b5400ac04e92156032336ffe8a22414b679 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -160,6 +160,18 @@ bool DiscardableSharedMemory::Unmap() {
   return true;
 }
 
+size_t DiscardableSharedMemory::LockedSize() const {
+  if (!shared_memory_.memory())
+    return 0;
+
+  SharedState result(subtle::NoBarrier_Load(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i));
+
+  return result.GetLockState() == SharedState::LOCKED
+             ? locked_page_count_ * base::GetPageSize()
+             : 0;
+}
+
 DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
     size_t offset, size_t length) {
   DCHECK_EQ(AlignToPageSize(offset), offset);