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

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index ddc3f2f54ac7c1ba90723d8ba99aabcdfbace0c3..e06a133dd8abe6709b19569a7db9d32fee883367 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -24,17 +24,11 @@
 #include "cc/scheduler/scheduler_state_machine.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;
+class SingleThreadTaskRunner;
 }
-class SingleThreadTaskRunner;
-}  // namespace base
 
 namespace cc {