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Fixing a bug where dragging text from one input element to another, and
then trying to undo this change, makes first input element's value
inaccessible. In order to fix this, the root editable element
associated with a `SelectionForUndoStep` is calculated at construction
time. It is important to do so at construction time as the base
position of the selection might be disconnected from the document, thus
we might not be able to compute the root editable element.

Bug: 864941
Test: Drag text from one input element to another, and then undo this
change. After undoing, try to use the first input element's `value`
property, and verify it has the correct value.

Change-Id: Ie0016f7459bad462e67e508b64e2686c39a66fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574954
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836325}