[ios] Modernize WebStateUserData & LazyWebStateUserData

It is possible to provide an inline implementation of UserDataKey()
and to remove the need for the WEB_STATE_USER_DATA_KEY_DECL() and
WEB_STATE_USER_DATA_KEY_IMPL() macros.

As a first step, provide the inline implementation and change the
macro so that it marks the variable kUserDataKey as unused to avoid
breaking the compilation. The macro will be removed in followup CLs.

Bug: 409299519
Change-Id: Id8fc174ba33ee104eeb5947441616390fb3310ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6441170
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1444758}