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..8b8bec2a7a62743393de3b2287c06b7dd2e06234 100644
--- a/net/disk_cache/entry_impl.cc
+++ b/net/disk_cache/entry_impl.cc
@@ -14,10 +14,14 @@
 #include "net/disk_cache/bitmap.h"
 #include "net/disk_cache/cache_util.h"
 #include "net/disk_cache/disk_format.h"
-#include "net/disk_cache/histogram_macros.h"
 #include "net/disk_cache/net_log_parameters.h"
 #include "net/disk_cache/sparse_control.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::TimeDelta;
 using base::TimeTicks;