In MultisammpleTest, a custom OSWindow object is created. Sometimes this
window's internal Objective-C window object is not released after the
test finishes, thus input events such as mouse/keyboard are still
forwarded to this window. However its content view is already released
causing the crash.

call [mWindow setContentView:nil] inside OSXWindow::destroy() seems to
fix this bug.

Bug: angleproject:4815
Change-Id: If00398ff0082624ade5e8be8ad3b0f91d17c23ed
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2281782
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>