Add support for platform views in the CanvasKit backend (flutter#14263)

* Add support for platform views in the CanvasKit backend

* Respond to comments

* Update license file

Author: Harry Terkelsen

ci/licenses_golden/licenses_flutter

@@ -386,6 +386,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/color_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/color_filter.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/embedded_views.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/engine_delegate.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/fonts.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/image.dart
@@ -394,6 +395,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/initialization.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/layer.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/layer_scene_builder.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/layer_tree.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/n_way_canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/path.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/path_metrics.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/picture.dart

lib/web_ui/lib/src/engine.dart

@@ -27,6 +27,7 @@ part 'engine/color_filter.dart';
 part 'engine/compositor/canvas.dart';
 part 'engine/compositor/canvas_kit_canvas.dart';
 part 'engine/compositor/color_filter.dart';
+part 'engine/compositor/embedded_views.dart';
 part 'engine/compositor/engine_delegate.dart';
 part 'engine/compositor/fonts.dart';
 part 'engine/compositor/image.dart';
@@ -35,6 +36,7 @@ part 'engine/compositor/initialization.dart';
 part 'engine/compositor/layer.dart';
 part 'engine/compositor/layer_scene_builder.dart';
 part 'engine/compositor/layer_tree.dart';
+part 'engine/compositor/n_way_canvas.dart';
 part 'engine/compositor/path.dart';
 part 'engine/compositor/path_metrics.dart';
 part 'engine/compositor/picture.dart';

lib/web_ui/lib/src/engine/compositor/canvas.dart

@@ -12,6 +12,10 @@ class SkCanvas {
 
   int get saveCount => skCanvas.callMethod('getSaveCount');
 
+  void clear(ui.Color color) {
+    skCanvas.callMethod('clear', <int>[color.value]);
+  }
+
   void clipPath(ui.Path path, bool doAntiAlias) {
     final SkPath skPath = path;
     final js.JsObject intersectClipOp = canvasKit['ClipOp']['Intersect'];