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The cl replaces the use of webm video with lottie animation in Assistant
opt-in flow.

Also update the style to center the lottie animation.

Bug: b/1012793
Test: Manual Test
Change-Id: I620d01dee2bf9c6670f57ec2390b6b0bf0da53df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891336Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712364}