[Mac] Move the reset of signal handlers to be very soon after the fork, befor...

[Mac] Move the reset of signal handlers to be very soon after the fork, before we do any mach IPC.

The MachPortSender constructor can sometimes hang forever (gets stuck in bootstrap_look_up()), so it is important to reset the child's signal handlers as early as possible.

Nico would like me to mention that this was his idea.

This is take two.  I couldn't reproduce the unit_tests failures either locally or on the release try bots.

BUG=35374
TEST=If in the forever backwards spinner state, closing the tab should not quit the browser.
TEST=In general, renderers and extensions and plugins should still work.
Review URL: http://codereview.chromium.org/3302009

TBR=rohitrao@chromium.org
Review URL: http://codereview.chromium.org/3322013

TBR=rohitrao@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=58572