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This fixes a crash if the download item controller goes away while the
button is tracking the mouse and then sends its action on mouse up.

Bug: 815161
Change-Id: I4ab377ca5fe190e3d95aa81b989c13c96b46f5b4
Reviewed-on: https://chromium-review.googlesource.com/957843
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542357}