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When TrayContainer's child preferredsize/visibility changed,
it wants to emit the PreferredSizeChanged() signal. However,
sometimes TrayContainer may not set the layout manager yet,
which does not update the preferred size and the signal will
be ignored.

I believe that ideally the current code structure/layout logic
should be refactored; the layout manager should always exist
for this view -- but this would be a good fix for the reported
issue of incorrect layout.

BUG=chromium:1100650
TEST=manually

Change-Id: Ifd64e9a8b6823366d54735951e3e4836d65b51d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284014Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786002}