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

cc/resources/bitmap_tile_task_worker_pool.h

Index: cc/resources/bitmap_tile_task_worker_pool.h
diff --git a/cc/resources/bitmap_tile_task_worker_pool.h b/cc/resources/bitmap_tile_task_worker_pool.h
index 20d57d247a1da647bcd61dc48f186c1653bdbe95..404a3a5f400f5e150060213778b3d6060624b367 100644
--- a/cc/resources/bitmap_tile_task_worker_pool.h
+++ b/cc/resources/bitmap_tile_task_worker_pool.h
@@ -11,16 +11,10 @@
 #include "cc/resources/tile_task_worker_pool.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 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;