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

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index e68bbc36f086c7ac1c1d5e5c58b88e87f1adf95a..dc58f52f7798636e8c5c85e5976a38d553f33701 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -19,10 +19,17 @@
 #include "cc/resources/ui_resource_client.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class TracedValue;
 }
+
+// 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::TracedValue;
 }
+}  // namespace base
 
 namespace cc {