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 e4344127c7643976dff8a2231d7ad44438bf0d1c..953fa13acbb37c77dcef11c8b79d297001657b23 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -43,10 +43,17 @@ class BoxF;
 }
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class ConvertableToTraceFormat;
 }
+
+// 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;
 }
+}  // namespace base
 
 namespace cc {