[tracing] Add method to create "weak" global dumps

base/trace_event/memory_allocator_dump.cc

Index: base/trace_event/memory_allocator_dump.cc
diff --git a/base/trace_event/memory_allocator_dump.cc b/base/trace_event/memory_allocator_dump.cc
index 5c5af7ee45842e716b92c5fcf7018641a3bfb023..f9b5799c05eb04b388e37dd6d7858d554990a65a 100644
--- a/base/trace_event/memory_allocator_dump.cc
+++ b/base/trace_event/memory_allocator_dump.cc
@@ -28,7 +28,8 @@ MemoryAllocatorDump::MemoryAllocatorDump(const std::string& absolute_name,
     : absolute_name_(absolute_name),
       process_memory_dump_(process_memory_dump),
       attributes_(new TracedValue),
-      guid_(guid) {
+      guid_(guid),
+      flags_(Flags::DEFAULT) {
   // The |absolute_name| cannot be empty.
   DCHECK(!absolute_name.empty());
 
@@ -90,6 +91,8 @@ void MemoryAllocatorDump::AsValueInto(TracedValue* value) const {
   value->BeginDictionaryWithCopiedName(absolute_name_);
   value->SetString("guid", guid_.ToString());
   value->SetValue("attrs", *attributes_);
+  if (flags_)
+    value->SetInteger("flags", flags_);
   value->EndDictionary();  // "allocator_name/heap_subheap": { ... }
 }