[ios] Improve page placeholder placement

Previously, the grey page placeholder image attempted to use the
webProxyView's contentInset property to properly position the
placeholder. This used frame setting.

The preferred way to position content in the BVC is to use the
ContentArea NamedGuide with autolayout contraints.

Using the preferred autolayout + NamedGuide approach also fixes a
placeholder malpositioning bug related to fullscreen experiments.

Bug: 956077
Change-Id: Ia3d115ae203bacb59d41cdb3c377a4a75c00dc95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593921
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657797}
4 files changed