Enable Chrome Upstream Save Card New Prompt Explanation

Bug: 836908
Change-Id: Idb2422e16651df75fe2dac30d0d4bc65410e67aa
Reviewed-on: https://chromium-review.googlesource.com/1031560Reviewed-by: Jared Saul <jsaul@google.com>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554475}

testing/variations/fieldtrial_testing_config.json

@@ -470,6 +470,24 @@
             ]
         }
     ],
+    "AutofillUpstreamUpdatePromptExplanation": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "AutofillUpstreamUpdatePromptExplanation"
+                    ]
+                }
+            ]
+        }
+    ],
     "AutofillUpstreamUseGooglePayOnAndroidBranding": [
         {
             "platforms": [