[PaymentHandler] Enable ScrollToExpandPaymentHandler from about flags

ScrollToExpandPaymentHandler can be enabled with a cmd flag, but not
from about://flags. In order to make it possible for users to enable
this feature from about://flags, this CL attaches this feature to
PaymentsExperimentalFeatures, which is one of about://flags, so that
enabling PaymentsExperimentalFeatures from either about://flags,
or cmd, will also enable this feature.

Bug: 999196

Change-Id: I28dae381fe2e4123db078f7b85addb6616ee24a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913163
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714994}

chrome/android/java/src/org/chromium/chrome/browser/payments/handler/PaymentHandlerCoordinator.java

@@ -8,6 +8,7 @@ import org.chromium.chrome.browser.ChromeActivity;
 import org.chromium.chrome.browser.ChromeFeatureList;
 import org.chromium.chrome.browser.ChromeVersionInfo;
 import org.chromium.chrome.browser.WebContentsFactory;
+import org.chromium.chrome.browser.payments.PaymentsExperimentalFeatures;
 import org.chromium.chrome.browser.payments.handler.toolbar.PaymentHandlerToolbarCoordinator;
 import org.chromium.chrome.browser.widget.bottomsheet.BottomSheetController;
 import org.chromium.components.embedder_support.view.ContentView;
@@ -90,6 +91,9 @@ public class PaymentHandlerCoordinator {
      *     solution.
      */
     public static boolean isEnabled() {
-        return ChromeFeatureList.isEnabled(ChromeFeatureList.SCROLL_TO_EXPAND_PAYMENT_HANDLER);
+        // Enabling the flag of either ScrollToExpand or PaymentsExperimentalFeatures will enable
+        // this feature.
+        return PaymentsExperimentalFeatures.isEnabled(
+                ChromeFeatureList.SCROLL_TO_EXPAND_PAYMENT_HANDLER);
     }
 }