GitLab

Updated Slider to be default constructable. Cleaned up some calling
code.

Bug: 1108460
Change-Id: I03b91286288322c2709e9ace2d0196dbc04ff230
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313496Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791234}