Revert of Move tracing namespace from base::debug to base::trace_event.

base/trace_event/trace_event_argument.cc

Index: base/trace_event/trace_event_argument.cc
diff --git a/base/trace_event/trace_event_argument.cc b/base/trace_event/trace_event_argument.cc
index 91ed9e85a23a0c026f7b1510b9e42c86b70257b2..00fcde16727d1bd4cf681d5e467050be24b67b92 100644
--- a/base/trace_event/trace_event_argument.cc
+++ b/base/trace_event/trace_event_argument.cc
@@ -8,7 +8,7 @@
 #include "base/values.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 
 TracedValue::TracedValue() : root_(new DictionaryValue()) {
   stack_.push_back(root_.get());
@@ -113,5 +113,5 @@
   DCHECK_EQ(1u, stack_.size()) << tmp;
 }
 
-}  // namespace trace_event
+}  // namespace debug
 }  // namespace base