chromeos: fix drag event coordinates

DragDropControllerMus was not converting the coordinates correct. In fixing this
it seems best to make this code consistent with the location of events passed
to clients.

BUG=920789
TEST=none

Change-Id: I5f1a86ac8038a8dd44dd66e7758821324028699f
Reviewed-on: https://chromium-review.googlesource.com/c/1412022
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623403}
15 files changed