[tracing] Introduce support for userland memory allocators dumps.

base/trace_event/memory_dump_manager.cc

Index: base/trace_event/memory_dump_manager.cc
diff --git a/base/trace_event/memory_dump_manager.cc b/base/trace_event/memory_dump_manager.cc
index 9a39faba8a645134200c8e7f979db2d9c2325374..8c97abf0de4327c7859cdc5d9cbc339155af15a5 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -139,12 +139,22 @@ void MemoryDumpManager::CreateLocalDumpPoint(DumpPointType dump_point_type,
   // deal with thread safety.
   {
     AutoLock lock(lock_);
+    MemoryAllocatorDump* last_allocator_dump = nullptr;
     for (auto it = dump_providers_enabled_.begin();
          it != dump_providers_enabled_.end();) {
       MemoryDumpProvider* dump_provider = *it;
       if (dump_provider->DumpInto(pmd.get())) {
         did_any_provider_dump = true;
         ++it;
+        // TODO(primiano): remove this once crbug.com/466121 gets fixed and
+        // the dump provider can inject the declarations for the allocator
+        // attributes just once when the trace is started (or ended).
+        MemoryAllocatorDump* allocator_dump = pmd->last_allocator_dump();
+        if (allocator_dump != last_allocator_dump) {
+          allocator_dump->set_extra_attributes_types(
+              dump_provider->allocator_attributes());
+          last_allocator_dump = allocator_dump;
+        }
       } else {
         LOG(ERROR) << "The memory dumper " << dump_provider->GetFriendlyName()
                    << " failed, possibly due to sandboxing (crbug.com/461788), "