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This CL is the 3rd option to resolve the crash of below call stack:
AppWindowLauncherItemController::GetAppWindow(aura::Window*, bool)
AppWindowLauncherItemController::SetActiveWindow(aura::Window*)
AppWindowLauncherController::OnWindowActivated()

One possible guessing is the AppWindowLauncherItemController object has
been deleted, but still used by AppServiceAppWindowLauncherController.
So modify the destruct function, and set the controller as null in
AppWindowBase.

BUG=1128491

Change-Id: Ied59f857653aa0a136cbe9f903ff79ce5856e34c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586647
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837244}