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Currently BrowserWindowController and BrowserFrameMac set some
delegates on the NSWindow object during construction for validating
and dispatching commands in the mainMenu bar (and other means).

These are not cleared upon destruction. However, AppKit may retain
NSWindows on NSEvents and things, resulting in validation or dispatch
attempts attempting to lookup a Browser* for the window and finding
null.

This CL adds plumbing in MacViews so that the NSWindow customisation
done at construction has a symmetric destruction opportunity as well.

Adds some test coverage to simulate the codepath that currently DCHECKs
that covers both Views and Cocoa browser windows.

Bug: 828738
Change-Id: I96f69c7912328abf84076f202c98617f4729f38a
Reviewed-on: https://chromium-review.googlesource.com/1068906Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561322}