[WebView] Disable PaymentMethodChangeEvent.

Before this patch, the PaymentMethodChangeEvent would be exposed in
WebView, because it has its own feature flag that was not explicitly
turned off in WebView.

This patch disables PaymentMethodChangeEvent and PaymentApp features in
WebView when the overarching feature PaymentRequest is also disabled.

After this patch, the PaymentMethodChangeEvent is not exposed in
WebView.

Bug: 965980
Change-Id: I4fb351a367638c9d830cfb189a5c9e8011c0a8eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625265
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662224}