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Require that the parent of a NativeWidgetMac always be either
- an NSView in the NSWindow of another NativeWidgetMac
- a nil NSView

Remove WidgetOwnerNSWindowAdapter (which enabled this functionality)
and BridgedNativeWidgetOwner (the interface it implements).

Update tests that happen to use this functionality so that they don't
use native view parents.

Bug: 859152
Change-Id: I69ea160852be5a44c7e2952445bd1115de72bd8e
Reviewed-on: https://chromium-review.googlesource.com/c/1258292
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596465}