[FedCM] Make RpContext an enum instead of a string

This avoids having to use string constants in an number of places.

Change-Id: I905a3f870dc07997b742df0a741e347b268804d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5678999
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323715}