Support nullable buffer for AudioBufferSourceNode.

Source/modules/webaudio/AudioBufferSourceNode.cpp

Index: Source/modules/webaudio/AudioBufferSourceNode.cpp
diff --git a/Source/modules/webaudio/AudioBufferSourceNode.cpp b/Source/modules/webaudio/AudioBufferSourceNode.cpp
index 9541cdb3377d309e529c88ff0a06cc5ef854c8e7..37437a2e3072ea6aea496f20b309a33f44832dac 100644
--- a/Source/modules/webaudio/AudioBufferSourceNode.cpp
+++ b/Source/modules/webaudio/AudioBufferSourceNode.cpp
@@ -58,6 +58,7 @@ PassRefPtr<AudioBufferSourceNode> AudioBufferSourceNode::create(AudioContext* co
 AudioBufferSourceNode::AudioBufferSourceNode(AudioContext* context, float sampleRate)
     : AudioScheduledSourceNode(context, sampleRate)
     , m_buffer(nullptr)
+    , m_nullableBuffer(nullptr)
     , m_isLooping(false)
     , m_loopStart(0)
     , m_loopEnd(0)
@@ -156,7 +157,10 @@ bool AudioBufferSourceNode::renderSilenceAndFinishIfNotLooping(AudioBus*, unsign
                 memset(m_destinationChannels[i] + index, 0, sizeof(float) * framesToProcess);
         }
 
-        finish();
+        // If buffer was set to null, it sould keep to render slience to ready rendering with proper buffer with setBuffer().
+        if (m_buffer != m_nullableBuffer)
+            finish();
+
         return true;
     }
     return false;
@@ -334,11 +338,19 @@ bool AudioBufferSourceNode::renderFromBuffer(AudioBus* bus, unsigned destination
 void AudioBufferSourceNode::setBuffer(AudioBuffer* buffer, ExceptionState& exceptionState)
 {
     ASSERT(isMainThread());
-    // FIXME: It does not look like we should throw if the buffer is null as
-    // the attribute is nullable in the specification.
+
+    // If a buffer is null, it is one channel of silence according to specification.
     if (!buffer) {
-        exceptionState.throwTypeError("buffer cannot be null");
-        return;
+        // Set mono channel, default frame = 512 and destination's sampleRate.
+        if (!m_nullableBuffer)
+            m_nullableBuffer = context()->createBuffer(1, 512, context()->sampleRate(), exceptionState);
+
+        if (!m_nullableBuffer) {
+            exceptionState.throwTypeError("failed creation of nullable buffer");
+            return;
+        }
+
+        buffer = m_nullableBuffer.get();
     }
 
     // The context must be locked since changing the buffer can re-configure the number of channels that are output.