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It is only used with mus. This also makes the class name match the file
name.

Also fix the build file so it only compiles when mus is enabled.

TBR=msw@chromium.org

Bug: none
Change-Id: I93dbc1a14051208bb0d5964fe68ec4d22201d2ce
Reviewed-on: https://chromium-review.googlesource.com/c/1495812Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636635}