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

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index 2f181fc7688a929f070799f2f93db58aec8decdb..206af66d7edff08522d3d35d89732a71eca6b35a 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -17,17 +17,11 @@
 #include "cc/base/cc_export.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 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;
+class SingleThreadTaskRunner;
 }
-class SingleThreadTaskRunner;
-}  // namespace base
 
 namespace gfx {
 class Rect;