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

cc/scheduler/scheduler_settings.h

Index: cc/scheduler/scheduler_settings.h
diff --git a/cc/scheduler/scheduler_settings.h b/cc/scheduler/scheduler_settings.h
index 4ff3c7eb5f7a3bcce279d38dd28d6b91e8dc4716..90c1a9c30f85b936cebecc786f0235bae668d351 100644
--- a/cc/scheduler/scheduler_settings.h
+++ b/cc/scheduler/scheduler_settings.h
@@ -11,10 +11,16 @@
 #include "cc/base/cc_export.h"
 
 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 {
 class LayerTreeSettings;