mock_launchd.cc: get rid of |throwaway_sockaddr_un| variable
mock_launchd.cc: get rid of |throwaway_sockaddr_un| variable

The constant |kMaxPipeNameLength| is not really necessary and
the sizeof can just be inlined in the base::strlcpy().

With this we can also get rid of |throwaway_sockaddr_un|
variable that was there just to make the constant.

BUG=None
R=tapted@chromium.org,mattm@chromium.org

Review URL: https://codereview.chromium.org/1772913002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#380393}

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