Check for already existing entry when adding to SiteProcessMap.

chrome/test/data/extensions/web_request_site_process_registration/test.js

Index: chrome/test/data/extensions/web_request_site_process_registration/test.js
diff --git a/chrome/test/data/extensions/web_request_site_process_registration/test.js b/chrome/test/data/extensions/web_request_site_process_registration/test.js
new file mode 100644
index 0000000000000000000000000000000000000000..a7f5d61ec5ac02a6abd6e22587f3006b7a37b26e
--- /dev/null
+++ b/chrome/test/data/extensions/web_request_site_process_registration/test.js
@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+chrome.webRequest.onBeforeRequest.addListener(
+  function(details) {
+    if (details.url == chrome.extension.getURL("/blocked.html")) {
+      return {cancel: true};
+    }
+  },
+  {urls: ["<all_urls>"]},
+  ["blocking"]
+);