startup: move some signals/flag-handling code into separate functions
startup: move some signals/flag-handling code into separate functions

I'm going to try to shrink ChromeMain() down to a bunch of function
calls, then move those functions into e.g. chrome_main_mac.cc.  Here's
one step on it.

TEST=compiles

Review URL: http://codereview.chromium.org/5166005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@66812 0039d316-1c4b-4281-b951-d872f2087c98

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