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

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index 7985a85e6287ed9840e71a4e14af72c71f93a660..40e295f78fc6e888f0828d98ec70b4b85ccfbf75 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -41,19 +41,13 @@
 #include "ui/gfx/transform.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 class ConvertableToTraceFormat;
 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::ConvertableToTraceFormat;
-using ::base::trace_event::TracedValue;
+class DictionaryValue;
 }
-class DictionaryValue;
-}  // namespace base
 
 namespace cc {