Get rid of last uses of deprecated toWebCoreString(v8::Handle<v8::Value>)

Source/bindings/v8/custom/V8OscillatorNodeCustom.cpp

 /*
  * Copyright (C) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 27 matching lines @@
 
     if (value->IsNumber()) {
         bool ok = false;
         uint32_t type = toUInt32(value, ok);
         if (!ok || !imp->setType(type))
             throwTypeError("Illegal OscillatorNode type", info.GetIsolate());
         return;
     }
 
     if (value->IsString()) {
-        String type = toWebCoreString(value);
+        String type = toWebCoreString(value.As<v8::String>());
         if (type == "sine" || type == "square" || type == "sawtooth" || type == "triangle") {
             imp->setType(type);
             return;
         }
     }
 
     throwTypeError("Illegal OscillatorNode type", info.GetIsolate());
 }
 
 } // namespace WebCore
 
 #endif // ENABLE(WEB_AUDIO)