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

cc/base/region.h

Index: cc/base/region.h
diff --git a/cc/base/region.h b/cc/base/region.h
index 294ea639200fde0035c8b25a9272404d95a52900..604e27b7c90ed6f5ff465b0c713b6e1442063994 100644
--- a/cc/base/region.h
+++ b/cc/base/region.h
@@ -15,10 +15,17 @@
 
 namespace base {
 class Value;
-namespace debug {
+namespace trace_event {
 class TracedValue;
 }
+
+// This is added temporarily. Once the renaming of namespace usages
+// accross all folders are done, this will be removed.
+// TODO(ssid): crrev.com/837303004
+namespace debug {
+using ::base::trace_event::TracedValue;
 }
+}  // namespace base
 
 namespace cc {
 class SimpleEnclosedRegion;