I'll check it.

Sorry, I haven't seen that there was an issue and that is was already assigned.
The test is flaky because sometimes addEventListener('message', ...) is called after the 'message' is received.

Sorry jinho. I hope you haven't started working on this. Here is my CL:
https://codereview.chromium.org/2785093003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/007c7242b1ac8aad353e2eac5279071ac59936c0

commit 007c7242b1ac8aad353e2eac5279071ac59936c0
Author: arthursonzogni <arthursonzogni@chromium.org>
Date: Fri Mar 31 07:58:15 2017

Fix flaky TEST PaymentAppBrowserTest.PaymentAppInvocation.

The test PaymentAppBrowserTest.PaymentAppInvocation was flaky. I was able to
make it fails several times. On the other side, when --browser-side-navigation
is enabled, this test fails very often.

This is due to a race condition:

payment_app.js                 ¦  payment_app_window.html
-------------------------------¦----------------------------------------
client.openWindow              ¦
        |      |_______________¦_______
        |                      ¦       |
 (promise onFullfiled called)  ¦       |
        |                      ¦       |
       ...                     ¦       |
        |                      ¦       |
        v                      ¦       |
  AddEventListener(...)        ¦ postMessage('payment_app_window_ready')

* postMessage('payment_app_window_ready') could be sended/received before
  the event listener is registered on the service worker. In this case
  the 'message' event was not handled. It causes the test to fails.

* A little bit surprising, it could also be send before the promise is
  resolved. That's why I had to use the maybeSendPaymentRequest().

BUG= 703602 

Review-Url: https://codereview.chromium.org/2785093003
Cr-Commit-Position: refs/heads/master@{#461074}

[modify] https://crrev.com/007c7242b1ac8aad353e2eac5279071ac59936c0/content/test/data/payments/payment_app.js

Thanks Arthur for taking care of this, and kudos for the awesome ASCII art in the commit message!