Add constant twoPiDouble/twoPiFloat to MathExtras.h

Source/platform/audio/EqualPowerPanner.cpp

Index: Source/platform/audio/EqualPowerPanner.cpp
diff --git a/Source/platform/audio/EqualPowerPanner.cpp b/Source/platform/audio/EqualPowerPanner.cpp
index d701db8d248ff5a90a81f2f0a5872d72b52a4c5d..1c7357bec5167c2933bc473d36678547b8fae670 100644
--- a/Source/platform/audio/EqualPowerPanner.cpp
+++ b/Source/platform/audio/EqualPowerPanner.cpp
@@ -101,8 +101,8 @@ void EqualPowerPanner::pan(double azimuth, double /*elevation*/, const AudioBus*
         }
     }
 
-    desiredGainL = cos(0.5 * piDouble * desiredPanPosition);
-    desiredGainR = sin(0.5 * piDouble * desiredPanPosition);
+    desiredGainL = cos(piOverTwoDouble * desiredPanPosition);
+    desiredGainR = sin(piOverTwoDouble * desiredPanPosition);
 
     // Don't de-zipper on first render call.
     if (m_isFirstRender) {