Set mash shelf width to available width.
Set mash shelf width to available width.

This makes the shelf view layout with the correct bounds.

BUG=57406
TEST=Overflow button appears for thin mash shell windows.
R=sky@chromium.org

Review URL: https://codereview.chromium.org/1750393004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378628}

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