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In the standard Wayland protocol, null should be allowed for the icon
argument of wl_data_device::start_drag. However, when nullptr was passed
to the |icon| of DragDropOperation, ScheduleStartDragDropOperation would
never be called, because couter_ assumed the icon was always present.
This CL fixes that issue.

TEST=manual
BUG=b:64963392

Change-Id: Ibb504096c31ed6e0048c6fcf7038190db9498fd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143177
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Fergus Dall <sidereal@google.com>
Cr-Commit-Position: refs/heads/master@{#758708}