[tracing] Introduce support for userland memory allocators dumps.

base/trace_event/memory_dump_manager.h

Index: base/trace_event/memory_dump_manager.h
diff --git a/base/trace_event/memory_dump_manager.h b/base/trace_event/memory_dump_manager.h
index 8a9b3b7e95a414dc72d3b17333199ff97d0bf21f..928de6f8e98c9c78354a07a5752674afc7e80d04 100644
--- a/base/trace_event/memory_dump_manager.h
+++ b/base/trace_event/memory_dump_manager.h
@@ -49,6 +49,12 @@ class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
   void OnTraceLogEnabled() override;
   void OnTraceLogDisabled() override;
 
+  // Returns the MemoryDumpProvider which is currently being dumping into a
+  // ProcessMemoryDump via DumpInto(...) if any, nullptr otherwise.
+  MemoryDumpProvider* dump_provider_currently_active() const {
+    return dump_provider_currently_active_;
+  }
+
  private:
   friend struct DefaultDeleter<MemoryDumpManager>;  // For the testing instance.
   friend struct DefaultSingletonTraits<MemoryDumpManager>;
@@ -69,6 +75,7 @@ class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
 
   std::vector<MemoryDumpProvider*> dump_providers_registered_;  // Not owned.
   std::vector<MemoryDumpProvider*> dump_providers_enabled_;     // Not owned.
+  MemoryDumpProvider* dump_provider_currently_active_;          // Now owned.
 
   // Protects from concurrent accesses to the |dump_providers_*|, e.g., tearing
   // down logging while creating a dump point on another thread.