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

cc/resources/zero_copy_tile_task_worker_pool.h

Index: cc/resources/zero_copy_tile_task_worker_pool.h
diff --git a/cc/resources/zero_copy_tile_task_worker_pool.h b/cc/resources/zero_copy_tile_task_worker_pool.h
index 8d3c0cfbd10120d69faca2523ac9142fc9c02908..4f3085c4c4b0f415aa6ba393f5314a9aadd6b565 100644
--- a/cc/resources/zero_copy_tile_task_worker_pool.h
+++ b/cc/resources/zero_copy_tile_task_worker_pool.h
@@ -11,10 +11,16 @@
 #include "cc/resources/tile_task_worker_pool.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class ConvertableToTraceFormat;
 }
+
+// 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;
 }
+}  // namespace base
 
 namespace cc {
 class ResourceProvider;