Handle OffscreenCanvas.convertToBlob exceptions: EncodingError & InvalidStateError

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 ea7dd1d06b1bde620311c0cc754ef06399d5fa75..a41f0c69a83ff962d9490753f956682b135ca320 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -305,7 +305,9 @@ ScriptPromise OffscreenCanvas::convertToBlob(ScriptState* scriptState,
     imageData = this->renderingContext()->toImageData(SnapshotReasonUnknown);
   }
   if (!imageData) {
-    return ScriptPromise();
+    exceptionState.throwDOMException(
+        InvalidStateError, "OffscreenCanvas object has no rendering contexts");
+    return exceptionState.reject(scriptState);
   }
 
   ScriptPromiseResolver* resolver = ScriptPromiseResolver::create(scriptState);