Initialize OpacityLayer's matrix to identity (#8467) (#8548)

Should have https://github.com/flutter/flutter/issues/30586 fixed once the engine rolls.

flow/layers/opacity_layer.cc

@@ -20,6 +20,13 @@ void OpacityLayer::EnsureSingleChild() {
   }
 
   auto new_child = std::make_shared<flow::TransformLayer>();
+
+  // Be careful: SkMatrix's default constructor doesn't initialize the matrix to
+  // identity. Hence we have to explicitly call SkMatrix::setIdentity.
+  SkMatrix identity;
+  identity.setIdentity();
+
+  new_child->set_transform(identity);
   for (auto& child : layers()) {
     new_child->Add(child);
   }

flow/layers/transform_layer.h

@@ -9,6 +9,8 @@
 
 namespace flow {
 
+// Be careful that SkMatrix's default constructor doesn't initialize the matrix
+// at all. Hence |set_transform| must be called with an initialized SkMatrix.
 class TransformLayer : public ContainerLayer {
  public:
   TransformLayer();