Get RootEditableElement from SelectionForUndoStep

Currently RootEditableElement is calculated by
doing EndingVisibleSelection, which do a layout and canonicalization
which is not required.
So we can use SelectionForUndoStep stored value to
get the RootEditableElement

BUG=657256

Change-Id: I3221979236058dd6107154bdbaaf0c5f21cab7af
Reviewed-on: https://chromium-review.googlesource.com/616376
Commit-Queue: Tanvir Rizvi <tanvir.rizvi@samsung.com>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495917}
5 files changed