Remove TODO in PaymentAppProviderImpl

This TODO was added to address cases where the web contents is destroyed
before explicitly closing the window. However, enforcing this would
violate encapsulation, e.g. the Java bottomsheet code shouldn't need
to worry about this nuance. See comment 20 on the bug for more detail.

Bug: 1099270
Change-Id: I66cc6fcbeefbdf16a9c9c6a1aa598ce0eb3f1b59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521779


Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Commit-Queue: Nick Burris <nburris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824921}