Move mojojs support out of fedcm-helper.js

That way, only tests that actually need mojojs will fail on other browsers.

R=yigu@chromium.org

Bug: 1309251
Change-Id: Ib0326fc28539afb18ae2717aa99f22fa6c05cccd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3792816


Commit-Queue: Yi Gu <yigu@chromium.org>
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1029928}