Add perf tests for starting Chrome with URL

tools/perf/page_sets/startup_pages_record.json

+{
+  "description": 
+  ["Pages to record data for testing starting Chrome with a URL. We can't use",
+   "startup_pages.json with record_wpr, since record_wpr requires a default", 
+   "navigate step, which we don't want for startup testing; but we do want to",
+   "record the pages it uses.",
+   "Also, record_wpr fails on about:blank, which we want to include in startup", 
+   "testing."],
+  "archive_data_file": "data/startup_pages.json",
+  "pages": [
+    {
+      "url": "http://news.bbc.co.uk",
+      "why": "typical page"
+    },
+    {
+      "url": "http://kapook.com",
+      "why": "Horribly complex page - stress test!"
+    }
+  ]
+}