Add the actual location coordinates to the touch cancel event

ui/aura/window_event_dispatcher.cc

Index: ui/aura/window_event_dispatcher.cc
diff --git a/ui/aura/window_event_dispatcher.cc b/ui/aura/window_event_dispatcher.cc
index 350066f7dd31e9e08c9ca72bf5f77377ab547e3e..6b36198ead19f98b77f09f89a341e3afcebfb81a 100644
--- a/ui/aura/window_event_dispatcher.cc
+++ b/ui/aura/window_event_dispatcher.cc
@@ -140,7 +140,6 @@ void WindowEventDispatcher::DispatchGestureEvent(ui::GestureEvent* event) {
   DispatchDetails details = DispatchHeldEvents();
   if (details.dispatcher_destroyed)
     return;
-
   Window* target = GetGestureTarget(event);
   if (target) {
     event->ConvertLocationToTarget(window(), target);
@@ -530,6 +529,11 @@ bool WindowEventDispatcher::CanDispatchToConsumer(
 }
 
 void WindowEventDispatcher::DispatchCancelTouchEvent(ui::TouchEvent* event) {
+  // We observe that the cancel touch event's location got scale twice,
+  // because the location we use to create the touch cancel event
+  // is already scaled, and we scale it again in the PrepareEventForDispatch
+  // function. Thus, we revert the location back in the below function.

[tdresser, 2014/08/15 14:22:04]

I think this could be a bit clearer. The key thing to mention is what units we
have now, and what units we need to convert to.

Perhaps:
The touchcancel event's location is based on the last known location of the
pointer, in dips. OnEventFromSource expects events with co-ordinates in raw
pixels, so we convert back to raw pixels here.

[sadrul, 2014/08/15 16:10:43]

What if you we don't transform the event in
WindowEventDispatcher::PrepareEventForDispatch() if the event was not created
from a native event (i.e. check event->HasNativeEvent() before doing the
transform)? Would that work?

[tdresser, 2014/08/15 16:48:44]

Yeah, that sounds reasonable.

[lanwei, 2014/08/15 17:09:28]

This is a very good suggestion, I will give it a try.

[lanwei, 2014/08/27 03:54:03]

Done.

+  event->UpdateForRootTransform(host_->GetRootTransform());
   DispatchDetails details = OnEventFromSource(event);
   if (details.dispatcher_destroyed)
     return;