Use a reasonable default pthread stack size on Mac OS X.
Use a reasonable default pthread stack size on Mac OS X.

Mac OS X uses a fixed default stack size of 512kB, which is too small for some
purposes. glibc on Linux uses ulimit -s, which is both a more reasonable thing
to do, and is generally set to a rational limit, such as 8MB.

BUG=43422
TEST=None really on the trunk. On the Mac 375 branch, using the .pac file on
     the bug, Chrome should not crash on startup.
Review URL: http://codereview.chromium.org/2035007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@46717 0039d316-1c4b-4281-b951-d872f2087c98

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