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This CL allows the iOS client to display desktop that is partially
obstructed by the notch while allowing user to pan the desktop out of
the obstructed area, similar to what we have done on the Android
client.

This CL also changes the previous logic that handles keyboard height.
We are now using DesktopViewport's SetSafeInsets() methods to adjust
for the keyboard rather than just changing the surface size.

Bug: 876014
Change-Id: I428cf2552acb3bc33cf932890a9395428d969a40
Reviewed-on: https://chromium-review.googlesource.com/1186027Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585666}