Add a new method to calculate the bounding box of drag drop target.
Add a new method to calculate the bounding box of drag drop target.

Update Keep code as well.

We have a bug 11768223. After digging into the problem, we think the root cause is ActiveDropTarget.box is not updated.

From the code, we only calculate the box when the drag starts. So here is the demonstration, suppose we have three items, https://docs.google.com/a/google.com/document/d/13sBqt4-cjoY7tR2v8Ea4_QUqGy62upK-4ZVDGa1nPh8/edit.

I did not find any method to update the box, and the targetList array, and ActiveDropTarget is private. I need to expose a method, so that I can update the ActiveDropTarget.box efficiently.

Repro case:

Drag the first item, move it two items down, to the third place. Then move the dragged item up. Notice the two items(2nd and 3rd) move together.
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