[iOS] Implement safe area fullscreen experiment.

When the safe area viewport adjustment mode is selected, the browser
container view will be laid out using the full bounds of the screen, and
the top safe area inset will be updated to push top-aligned fixed-
position elements into view.  Bottom-aligned fixed-position elements are
not affected by the safe area.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I8bbe41c3c8af2945cba1c17ad174ce42f78ae0f8
Reviewed-on: https://chromium-review.googlesource.com/1066784
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562056}

ios/chrome/browser/ui/BUILD.gn

@@ -364,6 +364,7 @@ source_set("ui_internal") {
     "//ios/chrome/browser/ui/find_bar",
     "//ios/chrome/browser/ui/first_run",
     "//ios/chrome/browser/ui/fullscreen",
+    "//ios/chrome/browser/ui/fullscreen:features",
     "//ios/chrome/browser/ui/fullscreen:ui",
     "//ios/chrome/browser/ui/history",
     "//ios/chrome/browser/ui/history_popup:coordinator",