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 faf4399fdca63b940aa59b4f123d339c7b9dcab0..95c108099a02309382f035f3a8ec58861cae33b9 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -24,11 +24,18 @@
 #include "cc/scheduler/scheduler_state_machine.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class ConvertableToTraceFormat;
 }
-class SingleThreadTaskRunner;
+
+// This is added temporarily. Once the renaming of namespace usages
+// accross all folders are done, this will be removed.
+// TODO(ssid): crrev.com/837303004
+namespace debug {
+using ::base::trace_event::ConvertableToTraceFormat;
 }
+class SingleThreadTaskRunner;
+}  // namespace base
 
 namespace cc {