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NSPopUpButtonCell in Mojave calls through the parent view at all
times, even during destruction. Work around this by properly
scoping the lifetime of the NSPopUpButtonCell in MenuButton, which
is something we weren't able to do in 10.5 but that was fixed
in 10.6 but that we never got around to updating.

BUG=869844

Change-Id: Iaecd978bc410810f02276f21ba13dd20ca73b195
Reviewed-on: https://chromium-review.googlesource.com/1159191
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580032}