Added clearLocalStorage method and test

Author: mvanbeusekom

packages/webview_flutter/webview_flutter_platform_interface/lib/v4/src/webview_controller_delegate.dart

@@ -151,6 +151,12 @@ abstract class WebViewControllerDelegate extends PlatformInterface {
         'clearCache is not implemented on the current platform');
   }
 
+  /// Clears the local storage used by the [WebView].
+  Future<void> clearLocalStorage() {
+    throw UnimplementedError(
+        'clearLocalStorage is not implemented on the current platform');
+  }
+
   /// Sets the [NavigationCallbackDelegate] containing the callback methods that
   /// are called during navigation events.
   Future<void> setNavigationCallbackDelegate(

packages/webview_flutter/webview_flutter_platform_interface/test/src/v4/webview_controller_delegate_test.dart

@@ -201,6 +201,20 @@ void main() {
     );
   });
 
+  test(
+      // ignore: lines_longer_than_80_chars
+      'Default implementation of clearLocalStorage should throw unimplemented error',
+      () {
+    final WebViewControllerDelegate controller =
+        ExtendsWebViewControllerDelegate(
+            const WebViewControllerCreationParams());
+
+    expect(
+      () => controller.clearLocalStorage(),
+      throwsUnimplementedError,
+    );
+  });
+
   test(
     'Default implementation of the setNavigationCallback should throw unimplemented error',
     () {