Add unisdt.h to make gcc 4.7 compile it

BUG=None
TEST=compile passed

Change-Id: I3c52e0c7b60572058d277c05ea05471327149601
Reviewed-on: https://gerrit.chromium.org/gerrit/21672
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>

metrics/metrics_library.h

@@ -7,6 +7,7 @@
 
 #include <sys/types.h>
 #include <string>
+#include <unistd.h>
 
 #include <base/memory/scoped_ptr.h>
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST