Introduced StoreKitTabHelper class

ios/chrome/browser/store_kit/store_kit_tab_helper.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_CHROME_BROWSER_STORE_KIT_STORE_KIT_TAB_HELPER_H_
+#define IOS_CHROME_BROWSER_STORE_KIT_STORE_KIT_TAB_HELPER_H_
+
+#include "base/macros.h"
+#import "ios/chrome/browser/store_kit/store_kit_launcher.h"
+#import "ios/web/public/web_state/web_state_user_data.h"
+
+// A Tab Helper object that can open to a page on iOS App Store for a given
+// app product ID.
+class StoreKitTabHelper : public web::WebStateUserData<StoreKitTabHelper> {
+ public:
+  explicit StoreKitTabHelper(web::WebState* web_state){};

[sdefresne, 2017/03/06 19:44:03]

Remove trailing semi-colon. You can also use = default here (for consistency
with the destructor).

BTW, I prefer when the constructor/destructor are in the implementation file if
it exists (this avoid potentially inlining it in every file that happens to use
this class). This is particularly important for C++ classes that have pointer to
Objective-C object (as the generated inlined constructor/destructor will have
different semantics if the file is included from a .mm file compiled with or
without ARC).

[pkl (ping after 24h if needed), 2017/03/07 01:17:54]

Moved implementation to .mm file.

+  ~StoreKitTabHelper() override = default;
+
+  void SetLauncher(id<StoreKitLauncher> launcher);
+  id<StoreKitLauncher> GetLauncher();
+
+  void OpenAppStore(NSString* app_id);
+
+ private:
+  id<StoreKitLauncher> store_kit_launcher_;  // weak

[sdefresne, 2017/03/06 19:44:03]

This is incorrect. The implementation file is compiled with ARC, so a raw
pointer to an Objective-C object is an owning pointer. If you want to make this
a non-owning pointer (a.k.a. weak pointer), you need to use "__weak".

However, __weak is unsupported if the header file is included in other file
compiled with ARC disabled, so until all our code uses ARC, the recommended way
is to use base::WeakNSProtocol<id<StoreKitLauncher>>. An alternative would be to
mark this as __unsafe_unretained (but as the name implies, this is unsafe).

Also, as base::WeakNSProtocol<> is self-documenting (as is __weak annotation),
you can remove the "// weak" comment.

[pkl (ping after 24h if needed), 2017/03/07 01:17:53]

Thanks for the explanation. Using base::WeakNSProtocol instead.

+
+  DISALLOW_COPY_AND_ASSIGN(StoreKitTabHelper);
+};
+
+#endif  // IOS_CHROME_BROWSER_STORE_KIT_STORE_KIT_TAB_HELPER_H_