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..e8f5265770860666a4e7d28dd984c2e79ae9b7a2 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -17,11 +17,18 @@
 #include "cc/base/cc_export.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class TracedValue;
 }
-class SingleThreadTaskRunner;
+
+// 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;
 }
+class SingleThreadTaskRunner;
+}  // namespace base
 
 namespace gfx {
 class Rect;