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This patch adds UMA to measure assistant container view's animation
smoothness.

Bug: b/113942916, 887583
Test: see UMA in histograms.
Change-Id: I83da71c4c0003061f5ca852a6128534a681646d3
Reviewed-on: https://chromium-review.googlesource.com/1234956Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593411}