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The logic to add FocusManager observers was smeared across
NativeWidgetMac and NativeWidgetMacNSWindowHost. Change this so that
all of the logic is in NativeWidgetMac::SetFocusManager. In the
process, cut-and-paste a handful of functionality over to
NativeWidgetMac.

This is part of the larger trend of merging NativeWidgetMac and
NativeWidgetMacNSWindowHost (preserving the latter perhaps, only as
mojo-implementing friend).

The motivation for this is that, when reparenting a non-top-level
NativeWidgetMac, we do not move the FocusManager observers, which
causes ConstrainedWindowViewTest.TabMoveTest to crash.

This patch should have no functional change. The next patch in the
sequence will update the observers as needed.

Bug: 957362
Change-Id: Id5a1d6183e446df647fa877239cb9dfb549e18a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983343Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727645}