Collect some histograms about signed binary downloads.

chrome/browser/safe_browsing/signature_util_posix.cc

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// This is a stub for the code signing utilities on Mac and Linux.
+// It should eventually be replaced with a real implementation.
+
+#include "chrome/browser/safe_browsing/signature_util.h"
+
+namespace safe_browsing {
+namespace signature_util {
+
+bool IsSigned(const FilePath& file_path) {
+  return false;
+}
+
+}  // namespace signature_util
+}  // namespace safe_browsing