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CHECK is failing because if the preview is a subset of the document and
is cancelled or fails after the subset is completed but while draft data
is still being generated, 2 messages are sent to the UI. Fix this issue
so that only 1 message is sent to the UI per preview request.

Bug: 740377
Change-Id: I751432f24e6283a55939a51cfe452f331da3f850
Reviewed-on: https://chromium-review.googlesource.com/571516
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487348}