Update legacy call to SkImage::makeSubset (flutter#43786)

In https://skia-review.googlesource.com/c/skia/+/671679 Skia updated the
calls to SkImage::makeSubset() to always need a `GrDirectContext*`. This
updates Flutter's one call that needed it.

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shell/platform/embedder/tests/embedder_unittests_util.cc

@@ -168,6 +168,7 @@ bool ImageMatchesFixture(const std::string& fixture_file_name,
   FML_CHECK(scene_image) << "Invalid scene image.";
 
   auto scene_image_subset = scene_image->makeSubset(
+      nullptr,
       SkIRect::MakeWH(fixture_image->width(), fixture_image->height()));
 
   FML_CHECK(scene_image_subset)