Un-nest sceneBuildDuration and windowRenderDuration in web SceneBuilderRecorder (#193260)

Splits out the
`dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart` fix from
https://github.com/flutter/flutter/pull/193135 per review discussion
(https://github.com/flutter/flutter/pull/193135#discussion_r3075077068).

Previously, `SceneBuilderRecorder.onDrawFrame` nested
`_profile!.recordAsync('windowRenderDuration', ...)` inside synchronous
`_profile!.record('sceneBuildDuration', ...)` without awaiting
`recordAsync`, which both triggered `unawaited_futures` and caused
`sceneBuildDuration` to include the synchronous portion of
`windowRenderDuration`. This PR un-nests the two recordings and makes
`onDrawFrame` `async` so `sceneBuildDuration` only measures
`sceneBuilder.build()` and `windowRenderDuration` is properly awaited.

dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart

diff --git a/dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart b/dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart
index 7b59500..32c33f4 100644
--- a/dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart
+++ b/dev/benchmarks/macrobenchmarks/lib/src/web/recorder.dart
@@ -269,17 +269,18 @@
         rethrow;
       }
     };
-    PlatformDispatcher.instance.onDrawFrame = () {
+    PlatformDispatcher.instance.onDrawFrame = () async {
       try {
-        _profile!.recordAsync('drawFrameDuration', () async {
+        await _profile!.recordAsync('drawFrameDuration', () async {
           final sceneBuilder = SceneBuilder();
           onDrawFrame(sceneBuilder);
-          _profile!.recordAsync('sceneBuildDuration', () async {
-            final Scene scene = sceneBuilder.build();
-            _profile!.recordAsync('windowRenderDuration', () async {
-              // On the web, render is asynchronous.
-              await (PlatformDispatcher.instance as dynamic).render(scene);
-            }, reported: false);
+          late final Scene scene;
+          _profile!.record('sceneBuildDuration', () {
+            scene = sceneBuilder.build();
+          }, reported: false);
+          await _profile!.recordAsync('windowRenderDuration', () async {
+            // On the web, render is asynchronous.
+            await (PlatformDispatcher.instance as dynamic).render(scene);
           }, reported: false);
         }, reported: true);
         endMeasureFrame();