Fix JavaScriptBlockingTestWebState constructor to initialize |observer_|

ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm

Index: ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm
diff --git a/ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm b/ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm
index d5b3b20e3cb938ee006dbdba4c10c02806578360..56de41edf9d61f341378f85cd63358b55722e614 100644
--- a/ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm
+++ b/ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util_unittest.mm
@@ -17,6 +17,7 @@
 // TestWebState subclass that allows simulating page loads.
 class JavaScriptBlockingTestWebState : public web::TestWebState {
  public:
+  JavaScriptBlockingTestWebState() : web::TestWebState(), observer_(nullptr) {}
   ~JavaScriptBlockingTestWebState() override {
     if (observer_)
       observer_->WebStateDestroyed();