Fix crash when calling convertToBlob on an OffscreenCanvas with no pixels

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 154 matching lines @@
   renderingContextFactories()[type] = std::move(renderingContextFactory);
 }
 
 bool OffscreenCanvas::originClean() const {
   return m_originClean && !m_disableReadingFromCanvas;
 }
 
 bool OffscreenCanvas::isPaintable() const {
   if (!m_context)
     return ImageBuffer::canCreateImageBuffer(m_size);
-  return m_context->isPaintable();
+  return m_context->isPaintable() && m_size.width() && m_size.height();
 }
 
 bool OffscreenCanvas::isAccelerated() const {
   return m_context && m_context->isAccelerated();
 }
 
 OffscreenCanvasFrameDispatcher* OffscreenCanvas::getOrCreateFrameDispatcher() {
   if (!m_frameDispatcher) {
     // The frame dispatcher connects the current thread of OffscreenCanvas
     // (either main or worker) to the browser process and remains unchanged
@@ skipping 54 matching lines @@
   return resolver->promise();
 }
 
 DEFINE_TRACE(OffscreenCanvas) {
   visitor->trace(m_context);
   visitor->trace(m_executionContext);
   EventTargetWithInlineData::trace(visitor);
 }
 
 }  // namespace blink