[web] Fix empty first frame in multiview mode (flutter#172493)

The issue was caused by a stale `EngineFlutterView.physicalSize` that
happened to be `Size(0, 0)` making rendering a no-op in the first frame.

Fixes flutter#172397

Author: mdebbar

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -197,6 +197,7 @@ class EngineFlutterView implements ui.FlutterView {
   /// the CSS box-model restrictions imposed on its `hostElement` (especially when
   /// hiding `overflow`). Flutter does not attempt to interpret the styles of
   /// `hostElement` to compute its `physicalConstraints`, only its current size.
+  @visibleForTesting
   void resize(ui.Size newPhysicalSize) {
     // The browser uses CSS, and CSS operates in logical sizes.
     final ui.Size logicalSize = newPhysicalSize / devicePixelRatio;
@@ -205,7 +206,7 @@ class EngineFlutterView implements ui.FlutterView {
       ..height = '${logicalSize.height}px';
 
     // Force an update of the physicalSize so it's ready for the renderer.
-    _computePhysicalSize();
+    _physicalSize = _computePhysicalSize();
   }
 
   /// Lazily populated and cleared at the end of the frame.

engine/src/flutter/lib/web_ui/test/engine/window_test.dart

@@ -744,6 +744,9 @@ Future<void> testMain() async {
       // Resize the host to 20x20 (physical pixels).
       view.resize(const ui.Size.square(50));
 
+      // The view's physicalSize should be updated too.
+      expect(view.physicalSize, const ui.Size(50.0, 50.0));
+
       await view.onResize.first;
 
       // The host tightly wraps the rootElement: