Add KVO compliant title property to CWVWebView.

ios/web_view/test/observer.mm

 // 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.
 
-#import "ios/web_view/test/boolean_observer.h"
+#import "ios/web_view/test/observer.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
-@implementation BooleanObserver
+@implementation Observer
 
 @synthesize keyPath = _keyPath;
 @synthesize lastValue = _lastValue;
 @synthesize object = _object;
 
 - (void)setObservedObject:(NSObject*)object keyPath:(NSString*)keyPath {
   [_object removeObserver:self forKeyPath:_keyPath];
 
   _lastValue = nil;
   _keyPath = [keyPath copy];
   _object = object;
-  if (keyPath) {
-    [_object addObserver:self
-              forKeyPath:_keyPath
-                 options:NSKeyValueObservingOptionNew
-                 context:nil];
-  }
+  [_object addObserver:self
+            forKeyPath:_keyPath
+               options:NSKeyValueObservingOptionNew
+               context:nil];
 }
 
 - (void)observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary<NSKeyValueChangeKey, id>*)change
                        context:(void*)context {
   if (![object isEqual:_object] || ![keyPath isEqualToString:_keyPath]) {
     // Ignore extraneous call from previous |_object| or |_keyPath|.
     return;
   }
   _lastValue = change[NSKeyValueChangeNewKey];
 }
 
 - (void)dealloc {
   [_object removeObserver:self forKeyPath:_keyPath];
 }
 
 @end