Add back a missing include in metrics_service.cc.

string_util.h is recursively included by one of the other includes in this file,
which is why we don't get a compile error, but it's best to always include what
you use.

BUG=None
TEST=None
Review URL: http://codereview.chromium.org/7065044

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86612 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/metrics/metrics_service.cc

@@ -163,6 +163,7 @@
 #include "base/md5.h"
 #include "base/metrics/histogram.h"
 #include "base/string_number_conversions.h"
+#include "base/string_util.h"  // For WriteInto().
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread.h"
 #include "base/utf_string_conversions.h"