Use PFFFT FFT library on Linux

Android uses the FFT library PFFFT.  Use this on Linux too.  Some
simple tests show that PFFFT is up to maybe 5% faster than the current
ffmpeg FFT library that was being used.

This also means that for all intents and purposes the Linux build is
the same as the Android build with respect to WebAudio.  Hence the
Linux bots should essentially produce the same coverage for Android.

There may also be some memory savings because PFFFT allows sharing the
FFT twiddle-factors and other state between FFTs of the same size.
FFmpeg doesn't allow this.

Bug: 968168
Change-Id: Ic453c857817bf49b664a6681ea4b194f4e66d5f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637717
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665212}

third_party/blink/renderer/config.gni

@@ -15,9 +15,8 @@ if (current_cpu == "arm") {
 
 declare_args() {
   # If true, use PFFFT for WebAudio FFT support.  This can be used for
-  # any Android architecture and also Linux and Windows.  We only use
-  # it on Android.
-  use_webaudio_pffft = is_android
+  # any Android architecture and also Linux and Windows.
+  use_webaudio_pffft = is_android || target_os == "linux"
 }
 
 declare_args() {