Remove WebStateUserData::kLocatorKey

This is to keep WebStateUserData API in sync with
WebContentsUserData (see https://crrev.com/c/1093015).

Bug: 589840
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I77db9c05167e91acafa15881ce85e43a9068c28e
Reviewed-on: https://chromium-review.googlesource.com/1151326Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579738}

ios/web/public/web_state/web_state_user_data.h

@@ -54,23 +54,15 @@ class WebStateUserData : public base::SupportsUserData::Data {
   }
 
  protected:
-  static inline void* UserDataKey() { return &kLocatorKey; }
-
- private:
-  // The user data key.
-  static int kLocatorKey;
+  static inline const void* UserDataKey() {
+    static const int kId = 0;
+    return &kId;
+  }
 };
 
-// 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 web::WebStateUserData<TYPE>::kLocatorKey = 0
+// Macro previously used to define the UserDataKey().
+// TODO(crbug.com/589840): Remove this once all use have been deleted.
+#define DEFINE_WEB_STATE_USER_DATA_KEY(TYPE)
 
 }  // namespace web