Add KVO compliant title property to CWVWebView.

ios/web_view/test/observers/boolean_observer.h

+// Copyright 2017 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.
+
+#ifndef IOS_WEB_VIEW_TEST_OBSERVERS_BOOLEAN_OBSERVER_H_
+#define IOS_WEB_VIEW_TEST_OBSERVERS_BOOLEAN_OBSERVER_H_
+
+#import "ios/web_view/test/observers/observer.h"
+
+// A specialized Observer for listening to a boolean property of an object.
+@interface BooleanObserver : Observer
+
+// The last value of performing |keyPath| on |object| after being notified of a
+// KVO value change or null if a change has not been observed. If |lastValue| is
+// nonnull it will be a NSNumber wrapped BOOL.
+@property(nonatomic, nullable, readonly) NSNumber* lastValue;
+
+@end
+
+#endif  // IOS_WEB_VIEW_TEST_OBSERVERS_BOOLEAN_OBSERVER_H_