[Payments] Add field trial testing config for JIT payment handler

Bug: 866996
Change-Id: I0c6a73956a2f31fe51bba193ec5348c2ef05bc5b
Reviewed-on: https://chromium-review.googlesource.com/1163634Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580927}

testing/variations/fieldtrial_testing_config.json

@@ -4693,6 +4693,25 @@
             ]
         }
     ],
+    "WebPaymentsJustInTimePaymentApp": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebPaymentsJustInTimePaymentApp"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebRTC-ApmGainController2Limiter": [
         {
             "platforms": [