[iOS] Expose the viewport insets in FullscreenController.

FullscreenModel knows enough information to calculate these insets, so
it makes sense for the calculation to occur there.  Exposing the
viewport insets in FullscreenController will allow chrome/ layer UI
code to easily account for viewport adjustments.

This API will be used to mask non-modal UIAlertController presentation.

Bug: 674649
Change-Id: I469cb133ef5d27bffcd4de5070bd12820514740e
Reviewed-on: https://chromium-review.googlesource.com/c/1339460
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612538}
8 files changed