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This CL adds an additional fullscreen viewport adjustment experiment.
In this approach, the WKWebView's frame is translated up and down to
account for the bottom inset created by the secondary toolbar.
Meanwhile, the top safe area inset is adjusted to push the top of the
rendered viewport below the toolbars.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Idb3ae9fb11ca2b35ee5219a52f9e08aa14363a7b
Reviewed-on: https://chromium-review.googlesource.com/1066975
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562562}