Remove DEFINE_WEB_STATE_USER_DATA_KEY from the example code.

Looks like the macro is deprecated per the comment at the bottom of the
file.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: If7763f67d427d03c150ec10b28a3d570ceef0f67
Reviewed-on: https://chromium-review.googlesource.com/c/1272758Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599570}

ios/web/public/web_state/web_state_user_data.h

@@ -25,8 +25,6 @@ namespace web {
 //   friend class web::WebStateUserData<Foo>;
 //   // ... more private stuff here ...
 // }
-// --- in foo.cc ---
-// DEFINE_WEB_STATE_USER_DATA_KEY(Foo);
 //
 template <typename T>
 class WebStateUserData : public base::SupportsUserData::Data {