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This callback may be reached as part of a close document action
after the engine has been destroyed. Further inspection of the code
shows that the same might also happen for Print().

Bug: 901640
Change-Id: I8faaeaf72adb37441f9e9c977093273c9e055e91
Reviewed-on: https://chromium-review.googlesource.com/c/1320810Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605829}