Change t.step() to t.step_func() from animations/prefixed

This is a follow-up CL of [1]. It changes t.step() to t.step_func()
from animations/prefixed and address review comments from [1].

[1] https://crrev.com/c/1525789

Bug: 936364
Change-Id: Ica1d58cd903ec8429f0ceac93a3ff3f1e514082b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530358
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642404}
12 files changed