[ios] Refactors SigninPromoView to use a UIStackView

- Changes the current SigninPromoView implementation to use a UIStackView.

Screenshots
Old:
https://drive.google.com/open?id=1k4_CJMGyM0shDfjmw3W4cfq1_6mTVLTD
https://drive.google.com/open?id=1AT7VnSVVPBcHJ-0FuTrpnbDQ2Xye5ByF
New:
https://drive.google.com/open?id=1fUECZTt5dXPjqNJtgxnfsLE0xWTvE5Fo
https://drive.google.com/open?id=1H1aWOm__89dG6YsjnHfSX1X29FAFp-li

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I275bd9c8af32507a1f484b34b44f58b72003c2c4
Reviewed-on: https://chromium-review.googlesource.com/1109579
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#569830}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c07c7d40103d252bcf09414a5e0b436126e5713b
1 file changed