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

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

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@58558 0039d316-1c4b-4281-b951-d872f2087c98

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