make sure tests and becnhmarks use correct backend (#72628)

dev/benchmarks/macrobenchmarks/README.md

@@ -121,10 +121,10 @@ Example:
 cd dev/benchmarks/macrobenchmarks
 
 # Runs in profile mode using the HTML renderer
-flutter run --profile -d web-server lib/web_benchmarks.dart
+flutter run --web-renderer=html --profile -d web-server lib/web_benchmarks.dart
 
 # Runs in profile mode using the CanvasKit renderer
-flutter run --dart-define=FLUTTER_WEB_USE_SKIA=true --profile -d web-server lib/web_benchmarks.dart
+flutter run --web-renderer=canvaskit --profile -d web-server lib/web_benchmarks.dart
 ```
 
 You can also run all benchmarks exactly as the devicelab runs them:

dev/bots/test.dart

@@ -995,6 +995,8 @@ Future<void> _runGalleryE2eWebTest(String buildMode, { bool canvasKit = false })
       'drive',
       if (canvasKit)
         '--dart-define=FLUTTER_WEB_USE_SKIA=true',
+      if (!canvasKit)
+        '--dart-define=FLUTTER_WEB_USE_SKIA=false',
       '--driver=test_driver/transitions_perf_e2e_test.dart',
       '--target=test_driver/transitions_perf_e2e.dart',
       '--browser-name=chrome',

dev/devicelab/lib/tasks/web_benchmarks.dart

@@ -29,6 +29,8 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
     await evalFlutter('build', options: <String>[
       'web',
       '--dart-define=FLUTTER_WEB_ENABLE_PROFILING=true',
+      if (!useCanvasKit)
+        '--dart-define=FLUTTER_WEB_USE_SKIA=false',
       if (useCanvasKit)
         '--dart-define=FLUTTER_WEB_USE_SKIA=true',
       '--profile',