Add constant twoPiDouble/twoPiFloat to MathExtras.h

Source/platform/audio/DownSampler.cpp

Index: Source/platform/audio/DownSampler.cpp
diff --git a/Source/platform/audio/DownSampler.cpp b/Source/platform/audio/DownSampler.cpp
index ca381e320b22c9c9f98191d85ea63748baf54a8b..c6ec18fe8f7a45ebbc7e5bb3eb8411d0b82b779a 100644
--- a/Source/platform/audio/DownSampler.cpp
+++ b/Source/platform/audio/DownSampler.cpp
@@ -73,7 +73,7 @@ void DownSampler::initializeKernel()
 
         // Compute Blackman window, matching the offset of the sinc().
         double x = static_cast<double>(i) / n;
-        double window = a0 - a1 * cos(2.0 * piDouble * x) + a2 * cos(4.0 * piDouble * x);
+        double window = a0 - a1 * cos(twoPiDouble * x) + a2 * cos(twoPiDouble * 2.0 * x);
 
         // Window the sinc() function.
         // Then store only the odd terms in the kernel.