Break out FFT function in ns_core
Break out FFT function in ns_core

This is done in order to make the code more readible and maintainable.
This introduces an error of only +1 and -1.

BUG=webrtc:3811
R=bjornv@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27749004

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@7512 4adac7df-926f-26a2-2b94-8c16560cd09d

Alkalyne/webrtctrunk,sippet/webrtc,krieger-od/webrtc,krieger-od/nwjs_chromium_webrtc,PersonifyInc/chromium_webrtc,Alkalyne/webrtctrunk,krieger-od/nwjs_chromium_webrtc,sippet/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,krieger-od/nwjs_chromium_webrtc,jchavanton/webrtc,PersonifyInc/chromium_webrtc,krieger-od/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,krieger-od/nwjs_chromium_webrtc,aleonliao/webrtc-trunk,krieger-od/nwjs_chromium_webrtc,jchavanton/webrtc,PersonifyInc/chromium_webrtc,krieger-od/webrtc,krieger-od/webrtc,sippet/webrtc,jchavanton/webrtc,krieger-od/webrtc,krieger-od/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,krieger-od/nwjs_chromium_webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,sippet/webrtc,PersonifyInc/chromium_webrtc,jchavanton/webrtc,PersonifyInc/chromium_webrtc,PersonifyInc/chromium_webrtc