Use nullptr instead of 0 in WebAudio

Source/platform/audio/AudioChannel.h

Index: Source/platform/audio/AudioChannel.h
diff --git a/Source/platform/audio/AudioChannel.h b/Source/platform/audio/AudioChannel.h
index a11f4e44591593417775334d05fe3367a91641ce..4067a6b562387cfdd01ae7d3ca9ec3ce23bd9a75 100644
--- a/Source/platform/audio/AudioChannel.h
+++ b/Source/platform/audio/AudioChannel.h
@@ -53,7 +53,7 @@ public:
     // Manage storage for us.
     explicit AudioChannel(size_t length)
         : m_length(length)
-        , m_rawPointer(0)
+        , m_rawPointer(nullptr)
         , m_silent(true)
     {
         m_memBuffer = adoptPtr(new AudioFloatArray(length));
@@ -62,7 +62,7 @@ public:
     // A "blank" audio channel -- must call set() before it's useful...
     AudioChannel()
         : m_length(0)
-        , m_rawPointer(0)
+        , m_rawPointer(nullptr)
         , m_silent(true)
     {
     }