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

content/common/host_discardable_shared_memory_manager.cc

Index: content/common/host_discardable_shared_memory_manager.cc
diff --git a/content/common/host_discardable_shared_memory_manager.cc b/content/common/host_discardable_shared_memory_manager.cc
index 7951ea127cc6238b4e207465cd5b8a9245a58e87..bfb689054948f6fd52da9897cd10805ea1821970 100644
--- a/content/common/host_discardable_shared_memory_manager.cc
+++ b/content/common/host_discardable_shared_memory_manager.cc
@@ -222,6 +222,16 @@ HostDiscardableSharedMemoryManager::AllocateLockedDiscardableMemory(
 bool HostDiscardableSharedMemoryManager::OnMemoryDump(
     const base::trace_event::MemoryDumpArgs& args,
     base::trace_event::ProcessMemoryDump* pmd) {
+  if (args.level_of_detail ==
+      base::trace_event::MemoryDumpLevelOfDetail::BACKGROUND) {
+    base::trace_event::MemoryAllocatorDump* total_dump =
+        pmd->CreateAllocatorDump("discardable");
+    total_dump->AddScalar(base::trace_event::MemoryAllocatorDump::kNameSize,
+                          base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+                          GetBytesAllocated());

[Primiano Tucci (use gerrit), 2016/06/02 19:31:41]

can you call this without the lock?

[ssid, 2016/06/03 00:27:47]

Hmm, we can only be stalling the memory-infra thread with this lock.
If we are holding lock, then it just reads the value and returns. So, we are not
stalling other threads.

+    return true;
+  }
+
   base::AutoLock lock(lock_);
   for (const auto& process_entry : processes_) {
     const int child_process_id = process_entry.first;