Correctly send dnd_finished events to delegate

The current implementation of wl::DataSource always calls its
delegate's OnDataSourceFinish() with the completed parameter set to
false. But for dnd_finished events, the parameter's value should be
true. This CL fixes this bug.

R=nickdiego@igalia.com

Bug: 1154835
Change-Id: I1340ffd7d0190ce2acd13b6e34c6af128bc2f30b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3250701
Commit-Queue: Max Ihlenfeldt <mihlenfeldt@igalia.com>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/main@{#936050}