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

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index db472c6aa3eda140e40f282358b791769c877ef0..9e8a9f5c62e83466e7090516dcf5d67038128f72 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -43,10 +43,16 @@ class BoxF;
 }
 
 namespace base {
-namespace debug {
+namespace trace_event {
 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 {