Add JankTracking to field trial testing config.

Bug: 581518
Change-Id: Icc8ef5e53428500a54695bf1a70cda139556ba67
Reviewed-on: https://chromium-review.googlesource.com/c/1349669Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611334}

testing/variations/fieldtrial_testing_config.json

@@ -2530,6 +2530,25 @@
             ]
         }
     ],
+    "JankTracking": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "JankTracking"
+                    ]
+                }
+            ]
+        }
+    ],
     "LegacySymantecPKI": [
         {
             "platforms": [