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After the menu closes, rendered content can be delivered
using a null |deliver_image_model_callback_|, so check for it
before calling.

Bug: 1134418
Change-Id: Ic97f3871a201a1c289835df74b891edf9d616460
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491141Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821297}