Passing in a promo string for the NewTabInProductHelp trial.

This modifies the trial in http://crrev.com/2a248c5

Bug: 734132
Change-Id: I6f0453465451023d33c8505f8b9f997004d5a094
Reviewed-on: https://chromium-review.googlesource.com/566335
Commit-Queue: Catherine Chung <catherinechung@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488816}

testing/variations/fieldtrial_testing_config.json

@@ -1884,7 +1884,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled",
+                    "name": "Enabled_1",
                     "params": {
                         "availability": "any",
                         "event_new_tab_opened": "name:new_tab_opened;comparator:==0;window:3650;storage:3650",
@@ -1892,7 +1892,8 @@
                         "event_session_time": "name:session_time;comparator:>=1;window:3650;storage:3650",
                         "event_trigger": "name:new_tab_trigger;comparator:any;window:3650;storage:3650",
                         "event_used": "name:new_tab_clicked;comparator:any;window:3650;storage:3650",
-                        "session_rate": "<=3"
+                        "session_rate": "<=3",
+                        "x_promo_string": "0"
                     },
                     "enable_features": [
                         "IPH_NewTab"