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There is no need for clients to call OnChildResizing() as it does nothing in
mus.

BUG=none
TEST=none

Change-Id: Ia38b2032068ac011325fdf0717e8db07a9943626
Reviewed-on: https://chromium-review.googlesource.com/c/1479290
Auto-Submit: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634718}