[bindings] Make sure v8::Isolate is being passed to toXXX() conversion routines as argument.

Source/modules/fetch/RequestInit.cpp

 // Copyright 2014 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.
 
 #include "config.h"
 #include "modules/fetch/RequestInit.h"
 
 #include "bindings/core/v8/Dictionary.h"
 #include "bindings/core/v8/V8ArrayBuffer.h"
 #include "bindings/core/v8/V8ArrayBufferView.h"
@@ skipping 59 matching lines @@
                 break;
             case FormDataElement::encodedFileSystemURL:
                 blobData->appendFileSystemURL(element.m_fileSystemURL, element.m_fileStart, element.m_fileLength, element.m_expectedFileModificationTime);
                 break;
             default:
                 ASSERT_NOT_REACHED();
             }
         }
         blobData->setContentType(AtomicString("multipart/form-data; boundary=", AtomicString::ConstructFromLiteral) + httpBody->boundary().data());
     } else if (body->IsString()) {
-        String stringValue(toUSVString(body, exceptionState));
+        String stringValue(toUSVString(isolate, body, exceptionState));
         blobData->appendText(stringValue, false);
         blobData->setContentType("text/plain;charset=UTF-8");
     } else {
         return;
     }
     const long long blobSize = blobData->length();
     bodyBlobHandle = BlobDataHandle::create(blobData.release(), blobSize);
 }
 
 }