[web] Set COEP:credentialless on flutter run/drive. (#152413)

This PR changes the value of the `COEP` response header from `require-corp` to `credentialless` in the Flutter tool, when running `flutter run`/`drive` for Web projects.

This enables running tests compiled to wasm, that access 3P resources (like Google Maps) without blocking those resources.

Docs:

* https://developer.chrome.com/blog/coep-credentialless-origin-trial#credentialless_to_the_rescue

## Issue

* flutter/flutter#152411

Author: ditman

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -1196,7 +1196,7 @@ class ReleaseAssetServer {
         'Access-Control-Allow-Origin': '*',
         if (_needsCoopCoep && _fileSystem.path.extension(file.path) == '.html') ...<String, String>{
           'Cross-Origin-Opener-Policy': 'same-origin',
-          'Cross-Origin-Embedder-Policy': 'require-corp',
+          'Cross-Origin-Embedder-Policy': 'credentialless',
         }
       });
     }
@@ -1208,7 +1208,7 @@ class ReleaseAssetServer {
         'Content-Type': 'text/html',
         if (_needsCoopCoep) ...<String, String>{
           'Cross-Origin-Opener-Policy': 'same-origin',
-          'Cross-Origin-Embedder-Policy': 'require-corp',
+          'Cross-Origin-Embedder-Policy': 'credentialless',
         },
       });
   }

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -64,7 +64,7 @@ shelf.Handler createDirectoryHandler(Directory directory, { required bool crossO
         if (needsCrossOriginIsolated)
           ...<String, String>{
             'Cross-Origin-Opener-Policy': 'same-origin',
-            'Cross-Origin-Embedder-Policy': 'require-corp',
+            'Cross-Origin-Embedder-Policy': 'credentialless',
           },
       },
     );
@@ -539,7 +539,7 @@ class FlutterWebPlatform extends PlatformPlugin {
         if (webRenderer == WebRendererMode.skwasm)
           ...<String, String>{
             'Cross-Origin-Opener-Policy': 'same-origin',
-            'Cross-Origin-Embedder-Policy': 'require-corp',
+            'Cross-Origin-Embedder-Policy': 'credentialless',
           }
       });
     }

packages/flutter_tools/test/general.shard/web/web_asset_server_test.dart

@@ -160,7 +160,7 @@ void main() {
     expect(response.statusCode, HttpStatus.ok);
     final Map<String, String> headers = response.headers;
     expect(headers['Cross-Origin-Opener-Policy'], 'same-origin');
-    expect(headers['Cross-Origin-Embedder-Policy'], 'require-corp');
+    expect(headers['Cross-Origin-Embedder-Policy'], 'credentialless');
   });
 
   testWithoutContext('release asset server serves html content without COOP/COEP headers when specified', () async {