canvas: make a temporary buffer when a context doesn't exist.

Source/core/frame/ImageBitmapTest.cpp

Index: Source/core/frame/ImageBitmapTest.cpp
diff --git a/Source/core/frame/ImageBitmapTest.cpp b/Source/core/frame/ImageBitmapTest.cpp
index 064c9ef74e8ce2433c81fe4572c72431c7ea15e9..4c26f683dc3edbbf1cbc910eda130d2a3a7c21c4 100644
--- a/Source/core/frame/ImageBitmapTest.cpp
+++ b/Source/core/frame/ImageBitmapTest.cpp
@@ -204,7 +204,8 @@ TEST_F(ImageBitmapTest, ImageResourceLifetime)
     canvasElement->setWidth(40);
     RefPtrWillBeRawPtr<ImageBitmap> imageBitmapDerived = nullptr;
     {
-        RefPtrWillBeRawPtr<ImageBitmap> imageBitmapFromCanvas = ImageBitmap::create(canvasElement.get(), IntRect(0, 0, canvasElement->width(), canvasElement->height()));
+        RefPtr<Image> canvasImage = canvasElement->copiedImage(BackBuffer);
+        RefPtrWillBeRawPtr<ImageBitmap> imageBitmapFromCanvas = ImageBitmap::create(canvasImage.get(), IntRect(0, 0, canvasElement->width(), canvasElement->height()));
         imageBitmapDerived = ImageBitmap::create(imageBitmapFromCanvas.get(), IntRect(0, 0, 20, 20));
     }
     CanvasRenderingContext* context = canvasElement->getContext("2d");