Introduce the WebState iOS provider interface.

ios/public/provider/web/web_state_user_data.h

Index: ios/public/provider/web/web_state_user_data.h
diff --git a/ios/public/provider/web/web_state_user_data.h b/ios/public/provider/web/web_state_user_data.h
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+++ b/ios/public/provider/web/web_state_user_data.h
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+// Copyright 2013 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_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_
+#define IOS_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_
+
+#include "ios/public/consumer/base/supports_user_data.h"
+#include "ios/public/provider/web/web_state.h"
+
+namespace ios {
+
+// A base class for classes attached to, and scoped to, the lifetime of a
+// WebState. For example:
+//
+// --- in foo.h ---
+// class Foo : public ios::WebStateUserData<Foo> {
+//  public:
+//   virtual ~Foo();
+//   // ... more public stuff here ...
+//  private:
+//   explicit Foo(ios::WebState* web_state);
+//   friend class ios::WebStateUserData<Foo>;
+//   // ... more private stuff here ...
+// }
+// --- in foo.cc ---
+// DEFINE_WEB_CONTENTS_USER_DATA_KEY(Foo);
+//
+template <typename T>
+class WebStateUserData : public ios::SupportsUserData::Data {
+ public:
+  // Creates an object of type T, and attaches it to the specified WebState.
+  // If an instance is already attached, does nothing.
+  static void CreateForWebState(WebState* web_state) {
+    if (!FromWebState(web_state))
+      web_state->SetUserData(UserDataKey(), new T(web_state));
+  }
+
+  // Retrieves the instance of type T that was attached to the specified
+  // WebState (via CreateForWebState above) and returns it. If no instance
+  // of the type was attached, returns NULL.
+  static T* FromWebState(WebState* web_state) {
+    return static_cast<T*>(web_state->GetUserData(UserDataKey()));
+  }
+  static const T* FromWebState(const WebState* web_state) {
+    return static_cast<const T*>(web_state->GetUserData(UserDataKey()));
+  }
+
+ protected:
+  static inline void* UserDataKey() {
+    return &kLocatorKey;
+  }
+
+ private:
+  // The user data key.
+  static int kLocatorKey;
+};
+
+// The macro to define the locator key. This key should be defined in the .cc
+// file of the derived class.
+//
+// The "= 0" is surprising, but is required to effect a definition rather than
+// a declaration. Without it, this would be merely a declaration of a template
+// specialization. (C++98: 14.7.3.15; C++11: 14.7.3.13)
+//
+#define DEFINE_WEB_STATE_USER_DATA_KEY(TYPE) \
+template<>                                      \
+int ios::WebStateUserData<TYPE>::kLocatorKey = 0
+
+}  // namespace ios
+
+#endif  // IOS_PUBLIC_PROVIDER_WEB_WEB_STATE_USER_DATA_H_