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This CL adds check to serial numbers passed to StartDrag and
SetSelection of DataDevice, so that serial numbers that do not exist or
serial numbers of invalid event types are ignored.

TEST=manual(Install Crostini and gedit, then confirm copy & paste /
drag & drop working)
BUG=746111

Change-Id: I914ced53a1d96590e24f4c4cb8ed74a43aac659f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255730Reviewed-by: Mitsuru Oshima (slow:gardening) <oshima@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781169}