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

base/trace_event/trace_event_impl_constants.cc

Index: base/trace_event/trace_event_impl_constants.cc
diff --git a/base/trace_event/trace_event_impl_constants.cc b/base/trace_event/trace_event_impl_constants.cc
index c46cf49fdb84cbc573a19dc41eaefc50a3745974..ffeacff38316c006eb0d70fb157838c3b7afb91d 100644
--- a/base/trace_event/trace_event_impl_constants.cc
+++ b/base/trace_event/trace_event_impl_constants.cc
@@ -5,7 +5,7 @@
 #include "base/trace_event/trace_event_impl.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 
 // Enable everything but debug and test categories by default.
 const char CategoryFilter::kDefaultCategoryFilterString[] = "-*Debug,-*Test";
@@ -24,5 +24,5 @@ const TraceLog::InternalTraceOptions
 const TraceLog::InternalTraceOptions
     TraceLog::kInternalRecordAsMuchAsPossible = 1 << 4;
 
-}  // namespace debug
+}  // namespace trace_event
 }  // namespace base