Disable UnixDomain[Client|Server]Socket on Fuchsia.

Previously the classes for Unix domain sockets were compiled on all
POSIX platforms, except NaCl, but the tests were disabled on iOS and
filtered on Fuchsia. This change disabled compilation of these classes
and unittests on all platforms on which they are not supported (iOS,
Fuchsia and NaCl).

Bug: 731302
Change-Id: I1da281845fd282d0f791cc4a324ef3d233f8a598
Reviewed-on: https://chromium-review.googlesource.com/664157
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501490}
2 files changed