[Payment Handler] Enable just-in-time install.

This patch enables the just-in-time installation of a payment handler
when merchant website calls PaymentRequest.show() with a user gesture.

Bug: 866996
Change-Id: I38b1ae7f672201f19ebe4011e22323ec5a572f36
Reviewed-on: https://chromium-review.googlesource.com/1190406Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587353}

components/payments/core/features.cc

@@ -28,6 +28,6 @@ const base::Feature kWebPaymentsSingleAppUiSkip{
     "WebPaymentsSingleAppUiSkip", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kWebPaymentsJustInTimePaymentApp{
-    "WebPaymentsJustInTimePaymentApp", base::FEATURE_DISABLED_BY_DEFAULT};
+    "WebPaymentsJustInTimePaymentApp", base::FEATURE_ENABLED_BY_DEFAULT};
 }  // namespace features
 }  // namespace payments

testing/variations/fieldtrial_testing_config.json

@@ -4955,25 +4955,6 @@
             ]
         }
     ],
-    "WebPaymentsJustInTimePaymentApp": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "WebPaymentsJustInTimePaymentApp"
-                    ]
-                }
-            ]
-        }
-    ],
     "WebRTC-Aec3AgcGainChangeResponseKillSwitch": [
         {
             "platforms": [