Provide an estimate of EngineLayer memory usage to the Dart GC (#6589)

See flutter/flutter#23171

Author: jason-simmons

lib/ui/painting/engine_layer.cc

@@ -17,6 +17,13 @@ namespace blink {
 
 EngineLayer::~EngineLayer() = default;
 
+size_t EngineLayer::GetAllocationSize() {
+  // Provide an approximation of the total memory impact of this object to the
+  // Dart GC.  The ContainerLayer may hold references to a tree of other layers,
+  // which in turn may contain Skia objects.
+  return 3000;
+};
+
 IMPLEMENT_WRAPPERTYPEINFO(ui, EngineLayer);
 
 #define FOR_EACH_BINDING(V)  // nothing to bind

lib/ui/painting/engine_layer.h

@@ -22,6 +22,9 @@ class EngineLayer : public RefCountedDartWrappable<EngineLayer> {
 
  public:
   ~EngineLayer() override;
+
+  size_t GetAllocationSize() override;
+
   static fml::RefPtr<EngineLayer> MakeRetained(
       std::shared_ptr<flow::ContainerLayer> layer) {
     return fml::MakeRefCounted<EngineLayer>(layer);