Clean up dependencies for disk_cache histogram_macros.h.

net/disk_cache/v3/backend_worker.cc

Index: net/disk_cache/v3/backend_worker.cc
diff --git a/net/disk_cache/v3/backend_worker.cc b/net/disk_cache/v3/backend_worker.cc
index be12a96eebedbd19b32fe545eba807a1b26044b4..a4acefb0c203c49d6c52f417aa92258a2640f91d 100644
--- a/net/disk_cache/v3/backend_worker.cc
+++ b/net/disk_cache/v3/backend_worker.cc
@@ -26,11 +26,11 @@
 #include "net/disk_cache/errors.h"
 #include "net/disk_cache/experiments.h"
 #include "net/disk_cache/file.h"
-
-// This has to be defined before including histogram_macros.h from this file.
-#define NET_DISK_CACHE_BACKEND_IMPL_CC_
 #include "net/disk_cache/histogram_macros.h"
 
+// Provide a BackendImpl object to macros from histogram_macros.h.
+#define CACHE_UMA_BACKEND_IMPL_OBJ this
+
 using base::Time;
 using base::TimeDelta;
 using base::TimeTicks;