Decouple MIDIAccess initialization from MIDIAccess class.

Source/modules/webmidi/MIDIAccessor.h

Index: Source/modules/webmidi/MIDIAccessor.h
diff --git a/Source/modules/webmidi/MIDIAccessor.h b/Source/modules/webmidi/MIDIAccessor.h
index 2c656ff6b25fc10da820059e6395faf832203917..d45870e3a94b5d7186d5707949d73fe4b5b8d1b0 100644
--- a/Source/modules/webmidi/MIDIAccessor.h
+++ b/Source/modules/webmidi/MIDIAccessor.h
@@ -48,6 +48,10 @@ public:
 
     void startSession();
     void sendMIDIData(unsigned portIndex, const unsigned char* data, size_t length, double timeStamp);
+    // MIDIAccessInitializer and MIDIAccess are both MIDIAccessClient.
+    // MIDIAccessInitializer is the first client and MIDIAccess takes over it
+    // once the initialization successfully finishes.
+    void setClient(MIDIAccessorClient* client) { m_client = client; }
 
     // blink::WebMIDIAccessorClient
     virtual void didAddInputPort(const blink::WebString& id, const blink::WebString& manufacturer, const blink::WebString& name, const blink::WebString& version) OVERRIDE;