Reland "Enable Safe Area compatible toolbar on iPhone X"

This CL enables the Safe Area compatible toolbar flag for iPhone X by
default.
In order to have the flag only enabled for iPhone X, it moves the
feature declaration to a protected file and add an utility function.

This reverts commit 21670fda4bdc2454f3c3f31dc7606eba0a17e2b8.

Bug: 778229
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I1ba581f1238a935b47bce2171d553ca759dbad5b
Reviewed-on: https://chromium-review.googlesource.com/757582
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514906}
18 files changed