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This CL exposes 'track', 'trackFill', 'thumb', and their variants
'trackTransitioning', 'trackFillTransitioning', 'thumbTransitionning'.

Bug: 972476
Change-Id: I807b35156cf0d0264dee5476583f22a01de7cfab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1687250
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674411}