Add SkColorSpace to Picture.toImage() (flutter/engine#3691)

Skia doesn't actually create the image if we don't supply a color space.

engine/src/flutter/lib/ui/painting/picture.cc

@@ -40,7 +40,7 @@ ftl::RefPtr<CanvasImage> Picture::toImage(int width, int height) {
   // TODO(abarth): We should pass in an SkColorSpace at some point.
   image->set_image(
       SkImage::MakeFromPicture(picture_, SkISize::Make(width, height), nullptr,
-                               nullptr, SkImage::BitDepth::kU8, nullptr));
+                               nullptr, SkImage::BitDepth::kU8, SkColorSpace::MakeSRGB()));
   return image;
 }