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

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 5471565a101f7d0baa2335f42b797144bf9479c0..a58b645c501f023aa851751690abb91b358060ba 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -19,16 +19,10 @@
 #include "cc/resources/ui_resource_client.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;
 }
-}  // namespace base
 
 namespace cc {