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Previously SetSelection did DCHECK for null DataSrouce, which was wrong.
According to Wayland document, we should clears the clipboard when null
DataSource is passed.

Bug; 789831
Test: exo_unittests

Change-Id: I9bff52cb4259b2d02faeb6a6e32353d0e3da4f2c
Reviewed-on: https://chromium-review.googlesource.com/798815
Commit-Queue: Daichi Hirono <hirono@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520775}