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On certain devices, the user will have an option to swipe from the shelf
on the last OOBE/first user run screen to end OOBE and enter the
session.

This adds a hook to login_screen interface that OOBE screen
implementation can use to enable gesture detection on login shelf, and
through which to get notified when the user swipes up.

Also, implements upward fling detection for login shelf (done by
login_shelf_gesture_controller, owned by shelf_widget) - when the
gesture detection is active, login shelf shows a drag handle with a
"contextual nudge" text above it.

Updates DragHandle API to support color/opacity changes, so the drag
handle appearance can be updated to match the login shelf controls on
OOBE and login screen.

BUG=976949

Change-Id: I670358c97de58c3f75ada9103df8098cc5074908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2073541Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745463}