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* Make more things const.
* Remove |starting_drag_| member by making |drag_start_point_| an
  Optional.
* Reduce nesting by reordering conditionals.

Bug: none
Change-Id: I1910ddf86af8d1545095f7b9a7e3d5c7c8ad6653
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255386Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780893}