Fix crash when calling convertToBlob on an OffscreenCanvas with no pixels

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

Index: third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
diff --git a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
index 7509d86c81e6e5077057d91abf0426c203f29d31..120f897de40d57dbd2c3ea58982cb774b8203ebb 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -172,7 +172,7 @@ bool OffscreenCanvas::originClean() const {
 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 {