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

content/child/child_thread.h

Index: content/child/child_thread.h
diff --git a/content/child/child_thread.h b/content/child/child_thread.h
index 24cfc61f22cbbaf37a51e700b1764c09dee9240b..7903d04309e11a0c2f9c4301f7f5dd5aafca70a3 100644
--- a/content/child/child_thread.h
+++ b/content/child/child_thread.h
@@ -21,8 +21,15 @@
 namespace base {
 class MessageLoop;
 
-namespace debug {
+namespace trace_event {
 class TraceMemoryController;
+}  // namespace trace_event
+
+// 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::TraceMemoryController;
 }  // namespace debug
 }  // namespace base