Update fieldtrial_testing_config.json for AutofillUpstreamEditableCardholderName

Bug: 848955
Change-Id: I4dbd64bd97a8483434fc247ca428dd4bacb4846d
Reviewed-on: https://chromium-review.googlesource.com/1176428Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: Jared Saul <jsaul@google.com>
Cr-Commit-Position: refs/heads/master@{#584234}

testing/variations/fieldtrial_testing_config.json

@@ -614,6 +614,24 @@
             ]
         }
     ],
+    "AutofillUpstreamEditableCardholderName": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "AutofillUpstreamEditableCardholderName"
+                    ]
+                }
+            ]
+        }
+    ],
     "AutofillUpstreamUpdatePromptExplanation": [
         {
             "platforms": [