[PaymentRequest] Replace '' with 'unknown' in PaymentComplete.

Replace the PaymentComplete enum value "" with the more descriptive "unknown".

Intent to implement:
 https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/B-8p5Ji-O9Q/nrkDOHHgBAAJ

related spec list:
 https://github.com/w3c/browser-payment-api/pull/312
 https://w3c.github.io/browser-payment-api/#dom-paymentcomplete

BUG=670291

Review-Url: https://codereview.chromium.org/2537493003
Cr-Commit-Position: refs/heads/master@{#435857}
1 file changed