[unified-consent] Center the sign-in screen on FRE on iPad.

This CL attempts to improve the layout of the sign-in screen on FRE
by centring the contents of the unified consent embedded on regular
width and regular height layouts.

Screenshots:
* Portrait: https://drive.google.com/file/d/1RpGHQlj6DqTxawpCNgbW_1fLAjR9CTEp/view?usp=sharing
* Landscape: https://drive.google.com/file/d/1qh16HAzNDttxWEGHepzK6RKjSSl3Nbe_/view?usp=sharing

Bug: 948679
Change-Id: I542c6f1dc6f683d7033bf83991e5f1b932226345
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549120
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646858}