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This removes parts of crrev.com/583350, preserving the following
- explicitly require that NSWindows be NativeWidgetMacNSWindow

This removes the ability to create an NSWindow in the AppShim process
for now. That will be re-added later.

This is a re-land of crrev.com/584323 (reverted in crrev.com/584323),
with the changes in lifetime management (use of autorelease) removed,
because it appears that tests make assumptions about lifetime. Added
a comment about this.

TBR=tapted

Bug: 859152
Change-Id: Ia0e2811c903316a0c1ed07be6cd7275bb8cf421d
Reviewed-on: https://chromium-review.googlesource.com/1180707Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584344}