Fix JavaScriptBlockingTestWebState constructor to initialize |observer_|

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/dialogs/javascript_dialog_blocking_util.h"
 
 #include "ios/web/public/load_committed_details.h"
 #include "ios/web/public/test/test_web_state.h"
 #include "ios/web/public/web_state/web_state_observer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 namespace {
 // TestWebState subclass that allows simulating page loads.
 class JavaScriptBlockingTestWebState : public web::TestWebState {
  public:
+  JavaScriptBlockingTestWebState() : web::TestWebState(), observer_(nullptr) {}
   ~JavaScriptBlockingTestWebState() override {
     if (observer_)
       observer_->WebStateDestroyed();
   }
 
   // Simulates a page load by sending a WebStateObserver callback.
   void SimulatePageLoaded() {
     web::LoadCommittedDetails details;
     if (observer_)
       observer_->NavigationItemCommitted(details);
@@ skipping 33 matching lines @@
 
 // Tests that ShouldBlockDialogsForPresenter() returns false after a  page load.
 TEST(JavaScriptDialogBlockingTest, StopBlocking) {
   JavaScriptBlockingTestWebState webState;
   EXPECT_FALSE(ShouldBlockJavaScriptDialogs(&webState));
   DialogBlockingOptionSelected(&webState);
   EXPECT_TRUE(ShouldBlockJavaScriptDialogs(&webState));
   webState.SimulatePageLoaded();
   EXPECT_FALSE(ShouldBlockJavaScriptDialogs(&webState));
 }