Fix mash shelf height.
Fix mash shelf height.

It was set to the height of the container (desktop).
Use the preferred size height instead.

BUG=557406
TEST=NONE
R=sky@chromium.org

Review URL: https://codereview.chromium.org/1573303002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368707}

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