Add proper includes for usage of the CityHash functions.

chrome/browser/metrics/extension_metrics.cc

Index: chrome/browser/metrics/extension_metrics.cc
diff --git a/chrome/browser/metrics/extension_metrics.cc b/chrome/browser/metrics/extension_metrics.cc
index 192aa5ea0599dd7a0560d17ea4cb2a0ecef369b4..b57a6f05d3b4bbc8c7cfe3de55f91339453e23cd 100644
--- a/chrome/browser/metrics/extension_metrics.cc
+++ b/chrome/browser/metrics/extension_metrics.cc
@@ -14,11 +14,7 @@
 #include "chrome/common/metrics/proto/system_profile.pb.h"
 #include "extensions/browser/extension_registry.h"
 #include "extensions/common/extension_set.h"
-
-// From third_party/smhasher/src/City.h; that file can't be included here due
-// to macro redefinitions (of UINT8_C, etc.) on Windows.
-// TODO(mvrable): Clean up City.h so it can be included directly.
-uint64 CityHash64(const char *buf, size_t len);
+#include "third_party/smhasher/src/City.h"
 
 namespace {