Use FFT from FFMpeg for OSX

Use FFT from FFMpeg for OSX

MacOS uses the vDSP library for the FFT routines.  Replace these with
the FFT from FFMpeg that is used on Linux and Windows.

Some measurements using http://jsfiddle.net/gMKh9/ (from
crbug.com/316740)as a benchmark. Test run using Macbook Pro 15" Retina
(Mid 2014)

vDSP:
413 ms
424 ms
428 ms
410 ms
432 ms

Avg: 421.4 ms

FFmpeg:
405 ms
401 ms
407 ms
425 ms
402 ms

Avg: 408 ms

About a 3% speed up.  Not much, but using the same FFT as for linux
and windows is good.

BUG=680617
TEST=none

[Raymond Toy, 2017-02-14 03:40:28]

In patch #2, we really remove support for OSX.  The new timing results are:

749 ms
730 ms
720 ms
727 ms
737 ms

Avg: 732 ms.

This is almost double the vDSP version.

There is too large a difference in performance to switch to ffmpeg.

[Raymond Toy, 2017-02-14 16:45:45]

Description was changed from

==========
Use FFT from FFMpeg for OSX

MacOS uses the vDSP library for the FFT routines.  Replace these with
the FFT from FFMpeg that is used on Linux and Windows.

Some measurements using http://jsfiddle.net/gMKh9/ (from
crbug.com/316740)as a benchmark. Test run using Macbook Pro 15" Retina
(Mid 2014)

vDSP:
413 ms
424 ms
428 ms
410 ms
432 ms

Avg: 421.4 ms

FFmpeg:
405 ms
401 ms
407 ms
425 ms
402 ms

Avg: 408 ms

About a 3% speed up.  Not much, but using the same FFT as for linux
and windows is good.

BUG=680617
TEST=none
==========

to

==========
Use FFT from FFMpeg for OSX

MacOS uses the vDSP library for the FFT routines.  Replace these with
the FFT from FFMpeg that is used on Linux and Windows.

Some measurements using http://jsfiddle.net/gMKh9/ (from
crbug.com/316740)as a benchmark. Test run using Macbook Pro 15" Retina
(Mid 2014)

vDSP:
413 ms
424 ms
428 ms
410 ms
432 ms

Avg: 421.4 ms

FFmpeg:
405 ms
401 ms
407 ms
425 ms
402 ms

Avg: 408 ms

About a 3% speed up.  Not much, but using the same FFT as for linux
and windows is good.

BUG=680617
TEST=none
==========