Remove opacity layer dcheck. (#46160)

In flutter/flutter#132849 it was found that rendering happens fine when this dcheck is removed.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: gaaclarke

flow/layers/opacity_layer.cc

@@ -36,8 +36,6 @@ void OpacityLayer::Diff(DiffContext* context, const Layer* old_layer) {
 }
 
 void OpacityLayer::Preroll(PrerollContext* context) {
-  FML_DCHECK(!layers().empty());  // We can't be a leaf.
-
   auto mutator = context->state_stack.save();
   mutator.translate(offset_);
   mutator.applyOpacity(SkRect(), DlColor::toOpacity(alpha_));

flow/layers/opacity_layer_unittests.cc

@@ -27,14 +27,6 @@ namespace testing {
 using OpacityLayerTest = LayerTest;
 
 #ifndef NDEBUG
-TEST_F(OpacityLayerTest, LeafLayer) {
-  auto layer =
-      std::make_shared<OpacityLayer>(SK_AlphaOPAQUE, SkPoint::Make(0.0f, 0.0f));
-
-  EXPECT_DEATH_IF_SUPPORTED(layer->Preroll(preroll_context()),
-                            "\\!layers\\(\\)\\.empty\\(\\)");
-}
-
 TEST_F(OpacityLayerTest, PaintingEmptyLayerDies) {
   auto mock_layer = std::make_shared<MockLayer>(SkPath());
   auto layer =