[FedCM][Refactor] Differentiate IdentityProviderDisplayData

This CL makes it easier to determine which variables are
of type IdentityProviderDisplayData and which are of type
IdentityProviderData by renaming the variables of type
IdentityProviderDisplayData. idp_data -> idp_display_data

The distinction between IdentityProviderDisplayData and
IdentityProviderData is important because FedCmAccountSelectionView
deals with both IdentityProviderDisplayData and IdentityProviderData.

This CL also converts IdentityProviderDisplayData to a struct and
converts the members to not end in underscores.

BUG=None

Change-Id: I87135c1e3089ef01fc0478f1e0e357164272378b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4194054
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1097119}