Gallery WebUI: replace paper-slider with cr-slider in image editor

Making cr-slider tabindex=0 when enabled so it can be focused using
|slider.focus()|. When a control is selected in the image editor, the
first control in the group is given focus (for example when clicking on
the brightness/contrast button, the brightness slider should have
focus.

Added css variables to allow for a different color scheme for image
editor sliders.

Bug: 902873
Change-Id: I4cf9c5082f81c9a8b50529c4e7f59537c4e4cfa6
Reviewed-on: https://chromium-review.googlesource.com/c/1357652
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615623}
7 files changed