Increase the open file limit on OS X to 1024

The default (256) is too low for pages that load a lot of resources all at
once. See https://codereview.chromium.org/125151 and bugs like
https://code.google.com/p/chromium/issues/detail?id=14137 and
https://code.google.com/p/chromium/issues/detail?id=151039. The new limit
matches what Chrome itself uses.