[tracing] Only add totals from discardable memory in background mode

content/child/child_discardable_shared_memory_manager.cc

Index: content/child/child_discardable_shared_memory_manager.cc
diff --git a/content/child/child_discardable_shared_memory_manager.cc b/content/child/child_discardable_shared_memory_manager.cc
index 1bb4ac5af4a0b44e0ea6d4d145336df9c79467b1..a0236a40ca7a2686a5dd86c80345455adc36ab3f 100644
--- a/content/child/child_discardable_shared_memory_manager.cc
+++ b/content/child/child_discardable_shared_memory_manager.cc
@@ -214,6 +214,21 @@ bool ChildDiscardableSharedMemoryManager::OnMemoryDump(
     const base::trace_event::MemoryDumpArgs& args,
     base::trace_event::ProcessMemoryDump* pmd) {
   base::AutoLock lock(lock_);
+  if (args.level_of_detail ==
+      base::trace_event::MemoryDumpLevelOfDetail::BACKGROUND) {
+    base::trace_event::MemoryAllocatorDump* total_dump =
+        pmd->CreateAllocatorDump("discardable");
+    const size_t total_size = heap_.GetSize();
+    const size_t freelist_size = heap_.GetSizeOfFreeLists();
+    total_dump->AddScalar(base::trace_event::MemoryAllocatorDump::kNameSize,
+                          base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+                          total_size - freelist_size);
+    total_dump->AddScalar("freelist_size",
+                          base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+                          freelist_size);
+    return true;
+  }
+
   return heap_.OnMemoryDump(pmd);
 }