[tracing] Fix single-process detection logic of ChildTraceMessageFilter

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 ac79144f9fdb27d7830c1b26c977bd952b1e5824..ec7651fe4447b72e4af9ab7bce31cbc5052b11b2 100644
--- a/base/trace_event/memory_dump_manager.cc
+++ b/base/trace_event/memory_dump_manager.cc
@@ -77,10 +77,7 @@ void MemoryDumpManager::Initialize() {
 
 void MemoryDumpManager::SetDelegate(MemoryDumpManagerDelegate* delegate) {
   AutoLock lock(lock_);
-  // TODO(primiano): The DCHECK below is disabled just temporary (and it is
-  // strongly useful). It should be re-enabled soon as crbug.com/474973 is fixed
-  // (ETA: end of April 2015). Commenting out just to de-entangle CL deps.
-  // DCHECK(delegate_ == nullptr);
+  DCHECK_EQ(static_cast<MemoryDumpManagerDelegate*>(nullptr), delegate_);
   delegate_ = delegate;
 }