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

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index ae067014f91c05350d42146a4d574501fbf74be3..5d668d9c40aa6ee6100a2d72b1453101fac3d80a 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -42,13 +42,19 @@
 #include "ui/gfx/transform.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class ConvertableToTraceFormat;
 class TracedValue;
 }
 
-class DictionaryValue;
+// 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::ConvertableToTraceFormat;
+using ::base::trace_event::TracedValue;
 }
+class DictionaryValue;
+}  // namespace base
 
 namespace cc {