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

cc/scheduler/delay_based_time_source.h

Index: cc/scheduler/delay_based_time_source.h
diff --git a/cc/scheduler/delay_based_time_source.h b/cc/scheduler/delay_based_time_source.h
index da968a6f017dca57e6e3b20882123e5f13197395..ea03f9d9742ed2297cba0008e589d10fc3107a68 100644
--- a/cc/scheduler/delay_based_time_source.h
+++ b/cc/scheduler/delay_based_time_source.h
@@ -12,17 +12,11 @@
 #include "cc/base/cc_export.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 class TracedValue;
 }
-
-// TODO(ssid): remove these aliases after the tracing clients are moved to the
-// new trace_event namespace. See crbug.com/451032. ETA: March 2015
-namespace debug {
-using ::base::trace_event::TracedValue;
+class SingleThreadTaskRunner;
 }
-class SingleThreadTaskRunner;
-}  // namespace base
 
 namespace cc {