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

content/child/child_thread_impl.h

Index: content/child/child_thread_impl.h
diff --git a/content/child/child_thread_impl.h b/content/child/child_thread_impl.h
index 20decd40739f5609b2d917bbc4684df6b147c1e7..6f1727afedfae9e93898be931c791d994c0d190f 100644
--- a/content/child/child_thread_impl.h
+++ b/content/child/child_thread_impl.h
@@ -22,8 +22,14 @@
 namespace base {
 class MessageLoop;
 
-namespace debug {
+namespace trace_event {
 class TraceMemoryController;
+}  // namespace trace_event
+
+// 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::TraceMemoryController;
 }  // namespace debug
 }  // namespace base