Add unittest to test CWVWebView.

ios/web_view/test/boolean_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"
+
+#include "base/logging.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation BooleanObserver
+
+@synthesize keyPath = _keyPath;
+@synthesize lastValue = _lastValue;
+@synthesize object = _object;
+
+- (void)setObservedObject:(NSObject*)object keyPath:(NSString*)keyPath {
+  if (_object) {
+    [_object removeObserver:self forKeyPath:_keyPath];
+  }
+
+  _lastValue = NO;
+  _keyPath = [keyPath copy];
+  _object = object;
+  [_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 != _object || keyPath != _keyPath) {
+    // Ignore extraneous call from previous |_object| or |_keyPath|.
+    return;
+  }
+
+  SEL selector = NSSelectorFromString(keyPath);
+  DCHECK([_object respondsToSelector:selector]);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
+  _lastValue = [object performSelector:selector];

[Eugene But (OOO till 7-30), 2017/05/19 18:39:48]

Is this something you can get from |change| dictionary?

[michaeldo, 2017/05/23 16:20:51]

Yes, it is cleaner to get out of |change|. Done.

+#pragma clang diagnostic pop
+}
+
+- (void)dealloc {
+  [_object removeObserver:self forKeyPath:_keyPath];
+}
+
+@end