Reinstate workarounds for atan2, fmod, pow in MathExtras on vs2013

I incorrectly removed some these workaround functions for VS2013 at
https://codereview.chromium.org/27009003 . The other non-workaround
functions (that just didn't exist previously) seem to be OK, but the
behaviour around +-Inf is still different than we expect for JS and
XPath.

R=jamesr@chromium.org
BUG=347870

Review URL: https://codereview.chromium.org/184103006

git-svn-id: svn://svn.chromium.org/blink/trunk@168231 bbb929c8-8fbe-4397-9dbb-9b2b20218538