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