[ios] Fixes broken constraints on TableView HeaderFooterViews.

Changes the headerFooterView padding constraints priorities
to prevent from constraints breaking.

Bug: 854117
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I08263cf6c692517ed16f9d331e9f49ae78186c29
Reviewed-on: https://chromium-review.googlesource.com/1107378
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578030}
4 files changed