[WebMIDI] MIDIManager::create should return non-null object.

media/midi/midi_manager.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_MIDI_MIDI_MANAGER_H_
 #define MEDIA_MIDI_MIDI_MANAGER_H_
 
 #include <set>
 #include <vector>
 
@@ skipping 47 matching lines @@
   void EndSession(MIDIManagerClient* client);
 
   // DispatchSendMIDIData() is called when MIDI data should be sent to the MIDI
   // system.
   // This method is supposed to return immediately and should not block.
   // |port_index| represents the specific output port from output_ports().
   // |data| represents a series of bytes encoding one or more MIDI messages.
   // |length| is the number of bytes in |data|.
   // |timestamp| is the time to send the data, in seconds. A value of 0
   // means send "now" or as soon as possible.
+  // The default implementation is for unsupported platforms.
   virtual void DispatchSendMIDIData(MIDIManagerClient* client,
                                     uint32 port_index,
                                     const std::vector<uint8>& data,
-                                    double timestamp) = 0;
+                                    double timestamp);
 
   // input_ports() is a list of MIDI ports for receiving MIDI data.
   // Each individual port in this list can be identified by its
   // integer index into this list.
   const MIDIPortInfoList& input_ports() { return input_ports_; }
 
   // output_ports() is a list of MIDI ports for sending MIDI data.
   // Each individual port in this list can be identified by its
   // integer index into this list.
   const MIDIPortInfoList& output_ports() { return output_ports_; }
 
  protected:
   // Initializes the MIDI system, returning |true| on success.
-  virtual bool Initialize() = 0;
+  // The default implementation is for unsupported platforms.
+  virtual bool Initialize();
 
   void AddInputPort(const MIDIPortInfo& info);
   void AddOutputPort(const MIDIPortInfo& info);
 
   // Dispatches to all clients.
   void ReceiveMIDIData(uint32 port_index,
                        const uint8* data,
                        size_t length,
                        double timestamp);
 
   bool initialized_;
 
   // Keeps track of all clients who wish to receive MIDI data.
   typedef std::set<MIDIManagerClient*> ClientList;
   ClientList clients_;
 
   // Protects access to our clients.
   base::Lock clients_lock_;
 
   MIDIPortInfoList input_ports_;
   MIDIPortInfoList output_ports_;
 
   DISALLOW_COPY_AND_ASSIGN(MIDIManager);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_MIDI_MIDI_MANAGER_H_