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Problem 1: the cross-fading was not executed properly. It should start by
drawing the attached bar at full opacity and transition to detached by
painting detached on top of that at partial opacity (defined by
animation state).

Problem 2: it painted as fully detached when current_state was 0. At this
point it should be painting as fully attached. This led to a single-frame
flash of lightness at the start of the attached->detached animation.

BUG=none

Review URL: https://codereview.chromium.org/1547563003

Cr-Commit-Position: refs/heads/master@{#367094}