Add perf tests for starting Chrome with URL

tools/perf/page_sets/startup_pages.json

+{
+  "description": [ 
+  "Pages for testing starting Chrome with a URL. ",
+  "Note that this file can't be used with record_wpr, since record_wpr ",
+  "requires a true navigate step, which we do not want for startup testing.",
+  "Instead use record_wpr startup_pages_record to record data for this test."],
+  "archive_data_file": "data/startup_pages.json",
+  "pages": [
+    {
+      "url": "about:blank",
+      "why": "typical page",
+      "navigate_steps" : [
+        {"action": "set_startup_url", "startup_url": "about:blank"},
+        {"action": "wait", "seconds": 10 }
+      ]
+    },
+    {
+      "url": "http://news.bbc.co.uk",
+      "why": "typical page",
+      "navigate_steps" : [
+        {"action": "set_startup_url", "startup_url": "http://news.bbc.co.uk"},
+        {"action": "wait", "seconds": 10 }
+      ]
+    },
+    {
+      "url": "http://kapook.com",
+      "why": "Horribly complex page - stress test!",
+      "navigate_steps" : [
+        {"action": "set_startup_url", "startup_url": "http://kapook.com"},
+        {"action": "wait", "seconds": 10 }
+      ]
+    }
+  ]
+}