Clean up dependencies for disk_cache histogram_macros.h.

net/disk_cache/backend_impl.cc

Index: net/disk_cache/backend_impl.cc
diff --git a/net/disk_cache/backend_impl.cc b/net/disk_cache/backend_impl.cc
index d10a19d9af09e76c553c99c9a54d07a0d8c30f6a..c934b87c8acd09bd8ce5394ada19e57b72a49be7 100644
--- a/net/disk_cache/backend_impl.cc
+++ b/net/disk_cache/backend_impl.cc
@@ -27,11 +27,11 @@
 #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;