Add constant twoPiDouble/twoPiFloat to MathExtras.h

Source/modules/webaudio/RealtimeAnalyser.cpp

Index: Source/modules/webaudio/RealtimeAnalyser.cpp
diff --git a/Source/modules/webaudio/RealtimeAnalyser.cpp b/Source/modules/webaudio/RealtimeAnalyser.cpp
index 24d7b729cabe902441ddda2c9099e0677199e03f..d7f671224a0b6512d22c7c771c487d7a526b1e73 100644
--- a/Source/modules/webaudio/RealtimeAnalyser.cpp
+++ b/Source/modules/webaudio/RealtimeAnalyser.cpp
@@ -138,7 +138,7 @@ void applyWindow(float* p, size_t n)
 
     for (unsigned i = 0; i < n; ++i) {
         double x = static_cast<double>(i) / static_cast<double>(n);
-        double window = a0 - a1 * cos(2 * piDouble * x) + a2 * cos(4 * piDouble * x);
+        double window = a0 - a1 * cos(twoPiDouble * x) + a2 * cos(twoPiDouble * 2.0 * x);
         p[i] *= float(window);
     }
 }