Use GURL instead of a plain string for the Wallet management URL

The goal of this refactoring is to slightly improve code clarity
by using a more specific type and to ensure consistency with the
handling of source URLs, which have been updated to use GURL in
crrev.com/c/8420529

Bug: b:560061580
Change-Id: I54dc7718e2a203fce942734efbf8df5a6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8427610
Commit-Queue: Thomas Schickinger <schickin@google.com>
Reviewed-by: Norge Vizcay <vizcay@google.com>
Reviewed-by: Vidhan Jain <vidhanj@google.com>
Cr-Commit-Position: refs/heads/main@{#1706771}