Reland "Add GPayAppDynamicUpdate feature flag."

This is a reland of 02d1e955

Original change's description:
> Add GPayAppDynamicUpdate feature flag.
>
> The feature flag controls the GPay native app integration for dynamic update on Android.
> If this feature flag is enabled, the GooglePayPaymentApp would enable a callback service and handle communications between the native GPay app and the browser for dynamic updates on shipping and payment data.
> Design doc is go/dynamic-update-mweb.
>
> Bug: 1074423
> Change-Id: I898199446179d6c314b833c3ecb8f017481fc596
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628745
> Commit-Queue: Kaiyu Chen <kylechenkyc@google.com>
> Reviewed-by: Sahel Sharify <sahel@chromium.org>
> Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#844274}

Bug: 1074423
Change-Id: Ic1cc2e3f4e6e3bff67246f0ce249196882a9b3f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637407Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Commit-Queue: Kaiyu Chen <kylechenkyc@google.com>
Cr-Commit-Position: refs/heads/master@{#845067}

components/payments/content/android/java/src/org/chromium/components/payments/PaymentFeatureList.java

@@ -18,6 +18,7 @@ public class PaymentFeatureList {
     /** Alphabetical: */
     public static final String ANDROID_APP_PAYMENT_UPDATE_EVENTS = "AndroidAppPaymentUpdateEvents";
     public static final String ENFORCE_FULL_DELEGATION = "EnforceFullDelegation";
+    public static final String GPAY_APP_DYNAMIC_UPDATE = "GPayAppDynamicUpdate";
     public static final String PAYMENT_REQUEST_SKIP_TO_GPAY = "PaymentRequestSkipToGPay";
     public static final String PAYMENT_REQUEST_SKIP_TO_GPAY_IF_NO_CARD =
             "PaymentRequestSkipToGPayIfNoCard";

components/payments/content/android/payment_feature_list.cc

@@ -27,6 +27,7 @@ const base::Feature* kFeaturesExposedToJava[] = {
     &features::kAppStoreBilling,
     &features::kAppStoreBillingDebug,
     &features::kEnforceFullDelegation,
+    &features::kGPayAppDynamicUpdate,
     &features::kPaymentRequestSkipToGPay,
     &features::kPaymentRequestSkipToGPayIfNoCard,
     &features::kReturnGooglePayInBasicCard,

components/payments/core/features.cc

@@ -89,5 +89,8 @@ const base::Feature kSecurePaymentConfirmation {
 #endif  // OS_MAC
 };
 
+const base::Feature kGPayAppDynamicUpdate{"GPayAppDynamicUpdate",
+                                          base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace payments

components/payments/core/features.h

@@ -90,6 +90,10 @@ extern const base::Feature kEnforceFullDelegation;
 // SecurePaymentConfirmation to be available.
 extern const base::Feature kSecurePaymentConfirmation;
 
+// If enabled, the GooglePayPaymentApp handles communications between the native
+// GPay app and the browser for dynamic updates on shipping and payment data.
+extern const base::Feature kGPayAppDynamicUpdate;
+
 }  // namespace features
 }  // namespace payments