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This patch implements a custom slider that provides the basic
functionality of the paper-slider but provides an additional option to
display a label for the current slider value. The slider has the Prefs
Behavior integrated with it.

This slider has a general implemention and is ready to be used in other
locations as well.

Functionalities implemented:
 - Slider works the same way as the paper-slider in terms of mouse and
   keyboard interactions.
 - Additionally The slider displays a label on mouse hover and keyboard
   focus.
 - The slider only works for discrete values or options.

This patch also adds web ui tests for all the slider functionalities.

A video of this slider in use can be found on the bug link below.

Bug: 798906
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I55fa98eb438ed1c48b4f40c14d8c05085e7a45b4
Component: Display Slider, display settings, webui tests
Reviewed-on: https://chromium-review.googlesource.com/985127
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547101}