Extract Textfield touch selection dragging into helper functions.

To make the logic in Textfield::OnGestureEvent a bit easier to follow,
move the touch selection dragging parts into helper functions. This
should help make the behaviour of selection_dragging_state_ and
selection_dragging_offset_ easier to understand since they are now only
used from the new helper functions.

Also add helper functions for touch scrolling to make it clearer how
touch selection dragging interacts with touch scrolling.

For reference, see how Textfield::OnGestureEvent looked before
selection dragging logic was added:
https://source.chromium.org/chromium/chromium/src/+/refs/tags/109.0.5414.85:ui/views/controls/textfield/textfield.cc;l=740;drc=ca87d6400e87419e0eaf902a1004c17c0430c4fa

Later, we could also consider moving the selection dragging code into a
separate class (or possibly combining some parts with
views::TouchSelectionControllerImpl or views::SelectionController).

Bug: b:244116654
Change-Id: I552ee1dabe93c3a077b86ba5ce36e21540bedf14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4592756
Commit-Queue: Michelle Chen <michellegc@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1156774}