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This CL aims to make the drag image while dragging tabs and tab groups
more consistent and more inline to each element's dimensions and DOM
structure by:

1) Initially basing offsets on the draggable target (the element with
the draggable attribute) instead of the element. TabGroupElements'
drag targets are their chips, which will result in different
calculations than if using the entire TabGroupElement's dimensions
since tab groups can have many tabs within them.

2) Using percentages instead of exact pixel values as the dimensions
of the drag image can change before and after dragstart.

3) Taking into consideration any margin or padding that a drag image
may have by using the dimensions of a drag image's "center" element.

Change-Id: I83c22a220023e6686ebea053a70c61f16c6239f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300988
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790028}