ipc: simplify SetupSocket() by reusing UnixDomainSocket functions
ipc: simplify SetupSocket() by reusing UnixDomainSocket functions

All the code in IPCChannelPosixTest::SetupSocket() is just
reimplementing the helper functions from unix_domain_socket_util.h. So
instead of reinveting the wheel, lets reuse this code here to siplify
the socket setup.

BUG=None
TEST=ipc_tests
R=cpu@chromium.org

Review URL: https://codereview.chromium.org/1647433003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372833}

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