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Crashes show a task coming from
-[NSWindow _endWindowBlockingModalSession:returnCode:] (before 10.13) or
NSWindowEndWindowModalSession() (10.13 and later). This attempts to access
the |modalDelegate| argument given to -[NSApp beginSheet:...], which is
not retained, and can be destroyed in some lifetimes.

The task is likely the one posted via an asynchronous Close() on a sheet,
attempting to close the sheet using [NSApp endSheet:]. If a parent window
of a sheet then triggers a more abrupt close, the sheet gets closed via
-[NSWindow close] as well. Normally, the delegate would be retained to
also end the sheet (to unblock the sheet machinery on the parent). However,
this only happens if the sheetParent is non-nil. It appears that under
some racy teardown paths the sheet's sheetParent can be nil at this point.

To fix, pass the NSWindow itself as |modalDelegate| to permit WeakPtr-like
semantics on the ViewsNSWindowDelegate. This allows some confusing
lifetimes in ViewsNSWindowDelegate to be tightened up, as well as cleanly
handle the mixed close codepaths that we have to cater for.

Bug: 851376
Change-Id: I1954dc76f7cff5dc3f54f879aaad51fec12b623f
Reviewed-on: https://chromium-review.googlesource.com/1096593
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567134}