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"
+
+#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) {

[Eugene But (OOO till 7-30), 2017/05/23 19:49:33]

No need for this check, it's perfectly fine to call a method on nil object.

[michaeldo, 2017/05/24 15:02:34]

Done.

+    [_object removeObserver:self forKeyPath:_keyPath];
+  }
+
+  _lastValue = NO;
+  _keyPath = [keyPath copy];
+  _object = object;
+  if (keyPath) {

[Eugene But (OOO till 7-30), 2017/05/23 19:49:33]

Is is there a value in passing nil keyPath? Should this be a DCHECK instead? 

[michaeldo, 2017/05/24 15:02:34]

It's the only current way to stop this object from observing, but maybe that
isn't a feature that is needed. I can annotate both params with nonnull in
header and not worry about nil keyPath. WDYT?

[Eugene But (OOO till 7-30), 2017/05/24 15:32:14]

I think if we need API for stopping the observing, it should be separate method.
Using |nonnull| sounds great.

+    [_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;
+  }
+  _lastValue = [change[NSKeyValueChangeNewKey] boolValue];
+}
+
+- (void)dealloc {
+  [_object removeObserver:self forKeyPath:_keyPath];
+}
+
+@end