Clean up dependencies for disk_cache histogram_macros.h.

net/disk_cache/eviction.cc

Index: net/disk_cache/eviction.cc
diff --git a/net/disk_cache/eviction.cc b/net/disk_cache/eviction.cc
index 47b52552ef5c4000282bd6a7dcd1f8a82396f664..578bda516e8273adec2b4c93fd5b153dd08e4f1d 100644
--- a/net/disk_cache/eviction.cc
+++ b/net/disk_cache/eviction.cc
@@ -38,9 +38,13 @@
 #include "net/disk_cache/disk_format.h"
 #include "net/disk_cache/entry_impl.h"
 #include "net/disk_cache/experiments.h"
-#include "net/disk_cache/histogram_macros.h"
 #include "net/disk_cache/trace.h"
 
+// Define BLOCKFILE_BACKEND_IMPL_OBJ to be a disk_cache::BackendImpl* in order
+// to use the CACHE_UMA histogram macro.
+#define BLOCKFILE_BACKEND_IMPL_OBJ backend_
+#include "net/disk_cache/histogram_macros.h"
+
 using base::Time;
 using base::TimeTicks;