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FullPanel has two child views which takes up 75% and 25% of the
total width. This can be done by box layout manager instead of
manually override Layout(). This CL refactors it into using
a layout manager.

BUG=1005568

Change-Id: Iaf01ebc6be64f449f793e60913bb1bf1ddca629f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848473
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704007}