Reland "Enable keyboard traversal in Payment Request UI."

This reverts commit 5254ea68efc6c8f6d9b631891eb8b13a37473e0b.

Reason for revert:
PaymentSheetViewControllerTest.ContinueIsNotAutoFocused flaked on some
Mac bots with the failure message indicating that the cancel button does
get focus on Mac. That assertion can be removed safely, because the main
purpose of that test case is to verify that the PAY_BUTTON is not
focused. That assertion has been holding.

Value of: GetByDialogViewID(DialogViewID::CANCEL_BUTTON)->HasFocus()
  Actual: false
Expected: true

This patch removes the assertion that CANCEL_BUTTON is focused.

Bug: 40897014 Original change's description:
> Revert "Enable keyboard traversal in Payment Request UI."
>
> This reverts commit 68d0c941aeafe6516ac9f4725d50114e96b7ca05.
>
> Reason for revert: causing flaky test in
> https://ci.chromium.org/ui/p/chromium/builders/ci/Mac13%20Tests
>
> Bug: 40897014, 414839152
> Original change's description:
> > 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}
>
> Bug: 40897014
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I89b6fba2455bf576d0b9892dfeb245988514dce9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6501036
> Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Nicolás Peña <npm@chromium.org>
> Owners-Override: Nicolás Peña <npm@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1454096}

Bug: 40897014, 414839152
Change-Id: Ia2af5470fc7685979527f29a116da5044f60d983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6501315
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454697}