[webview_flutter_platform_interface] Adds support for retrieving user agent (#4563)

Platform interface portion of https://github.com/flutter/packages/pull/4472

packages/webview_flutter/webview_flutter_platform_interface/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.5.0
+
+* Adds support to retrieve the user agent. See `PlatformWebViewController.getUserAgent`.
+
 ## 2.4.0
 
 * Adds support to retrieve the url from a web resource loading error. See `WebResourceError.url`.

packages/webview_flutter/webview_flutter_platform_interface/lib/src/platform_webview_controller.dart

@@ -270,6 +270,13 @@ abstract class PlatformWebViewController extends PlatformInterface {
       'setOnPermissionRequest is not implemented on the current platform',
     );
   }
+
+  /// Gets the value used for the HTTP `User-Agent:` request header.
+  Future<String?> getUserAgent() {
+    throw UnimplementedError(
+      'getUserAgent is not implemented on the current platform',
+    );
+  }
 }
 
 /// Describes the parameters necessary for registering a JavaScript channel.

packages/webview_flutter/webview_flutter_platform_interface/pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/flutter/packages/tree/main/packages/webview_flutt
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview_flutter%22
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 2.4.0
+version: 2.5.0
 
 environment:
   sdk: ">=2.18.0 <4.0.0"

packages/webview_flutter/webview_flutter_platform_interface/test/platform_webview_controller_test.dart

@@ -387,6 +387,19 @@ void main() {
       throwsUnimplementedError,
     );
   });
+
+  test(
+      'Default implementation of getUserAgent should throw unimplemented error',
+      () {
+    final PlatformWebViewController controller =
+        ExtendsPlatformWebViewController(
+            const PlatformWebViewControllerCreationParams());
+
+    expect(
+      () => controller.getUserAgent(),
+      throwsUnimplementedError,
+    );
+  });
 }
 
 class MockWebViewPlatformWithMixin extends MockWebViewPlatform