[ios] Make NewTabPageTabHelper::IsActive() private

When kCreateTabHelperOnlyForRealizedWebStates is enabled, the tab
helpers will only be created for unrealized WebState. Instead of
using NewTabPageTabHelper::IsActive() to decide whether a tab is
presenting the NTP (which will be incorrect if the tab helper has
not been created), use the function IsVisibleURLNewTabPage(...)
which will be made to work with unrealized WebStates in a followup
CL.

This is a no-op since IsVisibleURLNewTabPage(...) check that the
NewTabPageTabHelper is not null and then return the value of the
NewTabPageTabHelper::IsActive() method which is exactly what the
converted code is currently doing.

Bug: 409299519
Change-Id: I9243dd3ca267b5f4ae48d4fae0d4b109cce3ca15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6818280
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Commit-Queue: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1496395}