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Context:
Total is a field in the "new PaymentRequest()" API. It specifies the
amount and currency of the payment request. However, when the merchant
requests for the app-store billing (e.g., Google Play Store billing -
by specifying "https://play.google.com/billing" as the method), the
total field becomes unnecessary. This is because app-stores takes
the total from elsewhere.

Before:
The total field is mandatory for PaymentRequest.

After:
The total field is optional if only app-store methods are requested.
When total field is optional and left out, Chrome would add a total of
amount 0, currency "ZZZ" and label "AppStoreBillingTotalPlaceHolder".

Change:
* Added a RuntimeEnabledFeature: PaymentRequestTotalOptional
* Added an about flag: payment-request-optional-total
* change the optionality of the total field and details field of
PaymentRequest API.

Related Doc:
* Chrome Status: https://www.chromestatus.com/feature/5226111782879232
* Intent to Prototype: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/TJVn0Ps9ugA/3unr2Vo8AgAJ
* W3C explainer: https://github.com/w3c/payment-request/issues/912

Bug: 1066531

Change-Id: Id5ad87b9fc452fd41a1ebef066d981737545a235
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150974Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769914}