WebPayment: move Minimal UI into a separate about-flags

Context: currently, PaymentRequest API overzealously triggers the
Minimal UI when the WebPaymentExperimentalAPI flag is enabled.
It interferes with the development of the other experimental
features.

Old behavior: the Minimal UI feature is enabled when
the WebPaymentExperimentalAPI about-flag is enabled.

New behavior: enabling WebPaymentExperimentalAPI doesn't enable the
Minimal UI altogether. The Minimal UI has another
flag in about://flags called "Web Payment Minimal UI".

Change: move the Minimal UI feature out of the
WebPaymentExperimentalAPI flag into a separate flag.

Bug: 1000432
Change-Id: I7f77fd51ca22475123e68c652940dc8a282d748e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1967564
Commit-Queue: Liquan (Max) Gu <[email protected]>
Reviewed-by: Liquan (Max) Gu <[email protected]>
Reviewed-by: Rouslan Solomakhin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#724770}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 476dc3db..7e2f5195 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1325,6 +1325,9 @@
 extern const char kWebPaymentsExperimentalFeaturesName[];
 extern const char kWebPaymentsExperimentalFeaturesDescription[];
 
+extern const char kWebPaymentsMinimalUIName[];
+extern const char kWebPaymentsMinimalUIDescription[];
+
 extern const char kWebrtcCaptureMultiChannelApmName[];
 extern const char kWebrtcCaptureMultiChannelApmDescription[];