Extend JSBuiltinReducer to cover Math.abs as well.

R=titzer@chromium.org
TEST=compiler-unittests/JSBuiltinReducerTest.MathAbs

Review URL: https://codereview.chromium.org/605123004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24255 ce2b1a6d-e550-0410-aec6-3dcde31c8c00