I noticed a number of the WPT IDL tests are failing:

https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/external/wpt/payment-request/interfaces.https-expected.txt

FAIL PaymentRequest must be primary interface of new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL Stringification of new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "show" with the proper type (0) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "abort" with the proper type (1) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "canMakePayment" with the proper type (2) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "id" with the proper type (3) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "shippingAddress" with the proper type (4) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "shippingOption" with the proper type (5) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "shippingType" with the proper type (6) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "onshippingaddresschange" with the proper type (7) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."
FAIL PaymentRequest interface: new PaymentRequest([{supportedMethods: 'foo'}], {total: {label: 'bar', amount: {currency: 'BAZ', value: '0'}}}) must inherit property "onshippingoptionchange" with the proper type (8) assert_equals: Unexpected exception when evaluating object expected null but got object "TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence."

I think this is because of this IDL difference: 
Spec https://w3c.github.io/payment-request/#dom-paymentmethoddata:
dictionary PaymentMethodData {
    required DOMString supportedMethods;
    ...
};

Blink dictionary PaymentMethodData {
    required sequence<DOMString> supportedMethods;
    ...
};

Looks like the spec was changed a couple months ago here: https://github.com/w3c/payment-request/pull/551