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Replace GridLayout with BoxLayout and FlexLayout in cases where
GridLayout is not used as a true grid, but only for alignment.

This CL is part of the changes to apply documented best practice to
examples.

Bug: 897377
Change-Id: I789da2ebaa083f149079c36134dc8a1f48dd093a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316455
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791482}