Clean up dependencies for disk_cache histogram_macros.h.

net/disk_cache/v3/backend_impl_v3.cc

Index: net/disk_cache/v3/backend_impl_v3.cc
diff --git a/net/disk_cache/v3/backend_impl_v3.cc b/net/disk_cache/v3/backend_impl_v3.cc
index 7c85584726906a19947535fc5ab2dbc223e0e0b2..67dc86124a4cb825eba5b2032e3b61576a97122e 100644
--- a/net/disk_cache/v3/backend_impl_v3.cc
+++ b/net/disk_cache/v3/backend_impl_v3.cc
@@ -28,11 +28,11 @@
 #include "net/disk_cache/storage_block-inl.h"
 #include "net/disk_cache/v3/disk_format_v3.h"
 #include "net/disk_cache/v3/entry_impl_v3.h"
+#include "net/disk_cache/v3/histogram_macros.h"
 #include "net/disk_cache/v3/index_table.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/v3/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;