Set SO_REUSEADDR on layout test server sockets on non-Windows systems

Code to set the flag was removed as part of debugging crbug.com/378444
but this causes ports to report as "in use" for some time after the
test server stops, meaning that re-running layout tests with only a
short delay may fail.

Restore setting the flag, but scoped to non-Windows systems.

BUG=378444
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/464543003

git-svn-id: svn://svn.chromium.org/blink/trunk@180001 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/servers/server_base.py

@@ -261,7 +261,8 @@ class ServerBase(object):
     def _check_that_all_ports_are_available(self):
         for mapping in self._mappings:
             s = socket.socket()
-            # s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+            if not self._platform.is_win():
+                s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             port = mapping['port']
             try:
                 s.bind(('localhost', port))