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

cc/layers/layer_client.h

Index: cc/layers/layer_client.h
diff --git a/cc/layers/layer_client.h b/cc/layers/layer_client.h
index efdbe5105a11916268f72b3bc91870463ebb4038..4b98a959dcac4c35fea2aaaf614a0724045c75cd 100644
--- a/cc/layers/layer_client.h
+++ b/cc/layers/layer_client.h
@@ -11,16 +11,10 @@
 #include "cc/base/cc_export.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 class ConvertableToTraceFormat;
 }
-
-// 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::ConvertableToTraceFormat;
 }
-}  // namespace base
 
 namespace cc {