Implement suspend() and resume() for OfflineAudioContext

Source/modules/webaudio/AudioContext.h

 // Copyright 2015 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 AudioContext_h
 #define AudioContext_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "modules/webaudio/AbstractAudioContext.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Document;
 class ExceptionState;
 class ScriptState;
 
 // This is an AbstractAudioContext which actually plays sound, unlike an
 // OfflineAudioContext which renders sound into a buffer.
 class AudioContext : public AbstractAudioContext {
 public:
     static AbstractAudioContext* create(Document&, ExceptionState&);
 
     ~AudioContext() override;
     DECLARE_VIRTUAL_TRACE();
 
     ScriptPromise closeContext(ScriptState*) final;
-    bool isContextClosed() const final;
-
     ScriptPromise suspendContext(ScriptState*) final;
     ScriptPromise resumeContext(ScriptState*) final;
 
+    // This is to implement the pure virtual method from AbstractAudioContext.
+    // CANNOT be called on OfflineAudioContext.
+    ScriptPromise suspendContext(ScriptState*, double) final;
+
+    bool isContextClosed() const final;
     bool hasRealtimeConstraint() final { return true; }
 
 protected:
     AudioContext(Document&);
 
     void didClose() final;
 
 private:
     void stopRendering();
 
     unsigned m_contextId;
     RefPtrWillBeMember<ScriptPromiseResolver> m_closeResolver;
 };
 
 }
 
 #endif // AudioContext_h