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

content/browser/browser_main_loop.h

Index: content/browser/browser_main_loop.h
diff --git a/content/browser/browser_main_loop.h b/content/browser/browser_main_loop.h
index 661c1c56d9cb877b74c421e5c41f3ff100881613..176c4ba073a727f5becdc9c5535f4a82cf152724 100644
--- a/content/browser/browser_main_loop.h
+++ b/content/browser/browser_main_loop.h
@@ -23,9 +23,16 @@ class SystemMonitor;
 #if defined(OS_CHROMEOS)
 class MemoryPressureObserverChromeOS;
 #endif
-namespace debug {
+namespace trace_event {
 class TraceMemoryController;
 class TraceEventSystemStatsMonitor;
+}  // 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::TraceEventSystemStatsMonitor;
+using ::base::trace_event::TraceMemoryController;
 }  // namespace debug
 }  // namespace base