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

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180001

[jsbell, 2014-08-11 18:03:14]

dpranke@ - please take a look?

Per discussion over in https://codereview.chromium.org/413063002

(sorry for the delay, I was OOO for a week)

[Dirk Pranke, 2014-08-11 22:18:01]

lgtm. Sorry, I meant to get to this last week but didn't. Thanks!

[jsbell, 2014-08-11 22:28:59]

The CQ bit was checked by jsbell@chromium.org

[commit-bot: I haz the power, 2014-08-11 22:30:05]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jsbell@chromium.org/464543003/1

[commit-bot: I haz the power, 2014-08-11 22:34:17]

Change committed as 180001