Reland of 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 a58b645c501f023aa851751690abb91b358060ba..5471565a101f7d0baa2335f42b797144bf9479c0 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -19,10 +19,16 @@
 #include "cc/resources/ui_resource_client.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class TracedValue;
 }
+
+// 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::TracedValue;
 }
+}  // namespace base
 
 namespace cc {