Set relatedtarget for dragleave/enter events

third_party/WebKit/LayoutTests/external/wpt_automation/html/editing/dnd/events/relatedTarget-attribute-manual-automation.js

+importAutomationScript('/pointerevents/pointerevent_common_input.js');
+
+function inject_input() {
+    return mouseDragInTargets(['#draggable', '#outerdiv', '#innerdiv', '#outerdiv']);

[lanwei, 2017/05/05 20:27:51]

I looked at the code and confirmed with Sadrul, basically it is difficult to
make the drag and drop work for SyntheticPointerActions for all platforms.
Each platform has its own specific code doing drag and drop, like linux and
chromeOS, they use DesktopDragDropClientAuraX11::OnXdndDrop and
ash::DragDropController, Windows uses DesktopDropTargetWin::OnDragEnter. They
are taking low-level OS events, but SyntheticPointerActions is dispatched as
ui::mouseevent from SyntheticGestureTargetAura for mouse type. Now, we have to
use eventSender to generate drag and drop, it call
EventSender::DoDragAfterMouseUp directly.  

+}
+