mash: Move shelf menu code to //ash/common/shelf

It no longer has non-common dependencies.

I'll TBR since it's a purely mechanical file move.

BUG=615155
TEST=compiles
TBR=sky@chromium.org

Review-Url: https://codereview.chromium.org/2046563012
Cr-Commit-Position: refs/heads/master@{#398750}
10 files changed