[sync] Don't restrict to mainframe when exposing Javascript APIs

There are plans to invoke the Javascript APIs from an iframe embedded
in myaccount.google.com, to make sure that if the user changes their
account password, the new account password is properly added as a
recovery method, without counting on the less reliable server-side
integration (reauth).

Change-Id: I389d524a1800ec6ac13356a66268bc8a0781cb60
Bug: 1347763
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3779896
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1029119}