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According to https://wayland.freedesktop.org/docs/html/apa.html,
the drag source should consider the drag is successful when
wl_data_offer.finish is called, and unsuccessful when
wl_data_offer.destroy is called without finish.

TEST=DataDeviceTest.DataOfferNotFinished
BUG=b:64963392

Change-Id: I3338e1c1dff7f0860daa8f36bc2d5dcb0b30b873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206300Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771453}