Fix a small error in exception handling in convertToBlob

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

 // 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.
 
 #include "core/offscreencanvas/OffscreenCanvas.h"
 
 #include "core/dom/ExceptionCode.h"
 #include "core/fileapi/Blob.h"
 #include "core/html/ImageData.h"
 #include "core/html/canvas/CanvasAsyncBlobCreator.h"
@@ skipping 186 matching lines @@
     return exceptionState.reject(scriptState);
   }
 
   if (!this->originClean()) {
     exceptionState.throwSecurityError(
         "Tainted OffscreenCanvas may not be exported.");
     return exceptionState.reject(scriptState);
   }
 
   if (!this->isPaintable()) {
-    return ScriptPromise();
+    exceptionState.throwDOMException(
+        IndexSizeError, "The size of the OffscreenCanvas is zero.");
+    return exceptionState.reject(scriptState);
   }
 
   double startTime = WTF::monotonicallyIncreasingTime();
   String encodingMimeType = ImageEncoderUtils::toEncodingMimeType(
       options.type(), ImageEncoderUtils::EncodeReasonConvertToBlobPromise);
 
   ImageData* imageData = nullptr;
   if (this->renderingContext()) {
     imageData = this->renderingContext()->toImageData(SnapshotReasonUnknown);
   }
@@ skipping 17 matching lines @@
   return resolver->promise();
 }
 
 DEFINE_TRACE(OffscreenCanvas) {
   visitor->trace(m_context);
   visitor->trace(m_executionContext);
   EventTarget::trace(visitor);
 }
 
 }  // namespace blink