Enable keyboard traversal in Payment Request UI.

Before this patch, the Payment Request UI (both the browser payment
sheet and the payment handler sheet) could not be traversed by the
keyboard TAB key on Linux, Windows, and Chrome OS, because these
platforms do not allow focus on the content container view.

The inability of the TAB key to traverse the Payment Request UI had
reduced its accessibility and usability.

Mac OS was not affected, because it allows focusing on the content
container view.

Focusing on the "primary" button (e.g., "continue") is prohibited per
guidance on:
https://chromium.googlesource.com/chromium/src/+/main/docs/security/security-considerations-for-browser-ui.md
 - "Don't have a default-selected accept button."

This patch sets the default focus on the "secondary" button (e.g.,
"cancel"), if such button is present and enabled.

After this patch, payment flow UI on all desktop platforms can be
traversed with the TAB key.

Bug: 40897014
Change-Id: Id03031cde9607436019bee79067e0efa401f166c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6495535
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454006}