Fix fieldtrial_testing_config.json entry for reopen tab IPH

The enable_features array was missing in my last CL.

Bug: 887991
Change-Id: Ie69d9189a3255e93ee0fb41b24bc54db9b3d670a
Reviewed-on: https://chromium-review.googlesource.com/c/1489108Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635684}

testing/variations/fieldtrial_testing_config.json

@@ -4076,7 +4076,10 @@
                         "session_rate": "==0",
                         "x_new_tab_opened_timeout": "5",
                         "x_tab_minimum_active_duration": "60"
-                    }
+                    },
+                    "enable_features": [
+                        "IPH_ReopenTab"
+                    ]
                 }
             ]
         }