Fix a small error in exception handling in convertToBlob

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 550ae97f7812703f3bfd86bc1fb93d68371313ce..74d2988faa73344dbc3d3a83d460da634de4f2ea 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -204,7 +204,9 @@ ScriptPromise OffscreenCanvas::convertToBlob(ScriptState* scriptState,
   }
 
   if (!this->isPaintable()) {
-    return ScriptPromise();
+    exceptionState.throwDOMException(
+        IndexSizeError, "The size of the OffscreenCanvas is zero.");
+    return exceptionState.reject(scriptState);
   }
 
   double startTime = WTF::monotonicallyIncreasingTime();