Clean up dependencies for disk_cache histogram_macros.h.

net/disk_cache/entry_impl.cc

Index: net/disk_cache/entry_impl.cc
diff --git a/net/disk_cache/entry_impl.cc b/net/disk_cache/entry_impl.cc
index 4b6e4cf2b048a9f3ba227341c82ba416eda24944..d6796d70073675ac7344908ead0a009fdd78ba91 100644
--- a/net/disk_cache/entry_impl.cc
+++ b/net/disk_cache/entry_impl.cc
@@ -18,6 +18,9 @@
 #include "net/disk_cache/net_log_parameters.h"
 #include "net/disk_cache/sparse_control.h"
 
+// Provide a BackendImpl object to macros from histogram_macros.h.
+#define CACHE_UMA_BACKEND_IMPL_OBJ backend_
+
 using base::Time;
 using base::TimeDelta;
 using base::TimeTicks;