exo: Properly handle D&D when no icon is specified

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}

components/exo/drag_drop_operation.cc

@@ -114,9 +114,9 @@ DragDropOperation::DragDropOperation(
       base::BindOnce(&DragDropOperation::ScheduleStartDragDropOperation,
                      weak_ptr_factory_.GetWeakPtr());
 
-  // Make the count kMaxClipboardDataTypes + 1 so we can wait for the icon to be
-  // captured as well.
-  counter_ = base::BarrierClosure(kMaxClipboardDataTypes + 1,
+  // When the icon is present, make the count kMaxClipboardDataTypes + 1 so we
+  // can wait for the icon to be captured as well.
+  counter_ = base::BarrierClosure(kMaxClipboardDataTypes + (icon ? 1 : 0),
                                   std::move(start_op_callback));
 
   source->GetDataForPreferredMimeTypes(