Efficient and accessible Payment Handler UI updates

This patch updates the Payment Handler UI to prevent inefficient header
rebuilds that were previously triggered by color or text changes. These
rebuilds caused accessibility failures due to losing keyboard focus.

PaymentHandlerWebFlowViewController keeps track of navigations to
different pages, which may have different theme colors. If a page does
not define its own theme, then PaymentHandlerWebFlowViewController uses
Chrome's default theme color.

The new approach updates header elements in-place, which resolves the
accessibility issue.

Fixed: 418001134
Change-Id: Idbcdad8ad5d4c25263b2b067def9b4038f8dbfcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6732014
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1486947}