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The CL lets Surface observe drag and drop events and delegates the events to
DataDevice thorugh DataDeviceManager.

Bug: b:31988797
Test: None
Change-Id: I9574cd86137922d7527bd297d182a7b45e632c9e
Reviewed-on: https://chromium-review.googlesource.com/579250
Commit-Queue: Daichi Hirono <hirono@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491951}