[Payment Request][Desktop] Enable WebAuthn dialog in Payment Handler.

This patch creates a WebContentsModalDialogManager for the payment
handler renderer to allow the display of WebAuthn request dialog on top
of a payment handler window on desktop.

A new WebContentsModalDialogManagerDelegate subclass is introduced so
the dialog manager can interact with the WebContents. This delegate
borrows the WebContentsModalDialogHost from the browser for painting
modal dialogs at the correct position.

Bug: 938491
Change-Id: I67e625296c698b511bc6c0650975e60e7c61e245
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531303
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642933}
5 files changed