[Mac] Make switch-N crash keys 1-based.
[Mac] Make switch-N crash keys 1-based.

To match other platforms.

BUG=60991
TEST=Monitor crash server.


Review URL: http://codereview.chromium.org/8055028

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@103538 0039d316-1c4b-4281-b951-d872f2087c98

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