mac: Use stabs instead of dwarf when fastbuild=1
mac: Use stabs instead of dwarf when fastbuild=1

libbase_i18n.a goes from 1MB to 300kB with this. I could imagine that
this CL makes the trybot compile time quite a bit faster.

BUG=none
TEST=none
TBR=mark

Review URL: http://codereview.chromium.org/7737027

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@99673 0039d316-1c4b-4281-b951-d872f2087c98

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