[web] Reland - Switch web-render option default to auto (#23454)

Author: ferhatb

lib/web_ui/dev/test_runner.dart

@@ -619,6 +619,8 @@ class TestCommand extends Command<bool> with ArgUtils {
       '--enable-experiment=non-nullable',
       '--no-sound-null-safety',
       if (input.forCanvasKit) '-DFLUTTER_WEB_USE_SKIA=true',
+      if (!input.forCanvasKit) '-DFLUTTER_WEB_AUTO_DETECT=false',
+      if (!input.forCanvasKit) '-DFLUTTER_WEB_USE_SKIA=false',
       '-O2',
       '-o',
       targetFileName, // target path.

lib/web_ui/lib/src/engine/canvaskit/initialization.dart

@@ -28,10 +28,10 @@ bool _detectRenderer() {
 
 /// Auto detect which rendering backend to use.
 ///
-/// Using flutter tools option "--web-render=auto" would set the value to true.
-/// Otherwise, it would be false.
+/// Using flutter tools option "--web-render=auto" or not specifying one
+/// would set the value to true. Otherwise, it would be false.
 const bool _autoDetect =
-    bool.fromEnvironment('FLUTTER_WEB_AUTO_DETECT', defaultValue: false);
+    bool.fromEnvironment('FLUTTER_WEB_AUTO_DETECT', defaultValue: true);
 
 /// Enable the Skia-based rendering backend.
 ///