Reland "Reland "Adding `OnEnterBackForwardCache` to `PageAnchorsMetricsObserver`""

This is a reland of commit 147214e30309b26bd245fb8c5f46f7b0848ab54f

Original change's description:
> Reland "Adding `OnEnterBackForwardCache` to `PageAnchorsMetricsObserver`"
>
> This is a reland of commit b4cfa991ed36ba4d0a74f37bd0064f3134d0373b
>
> Original change's description:
> > Adding `OnEnterBackForwardCache` to `PageAnchorsMetricsObserver`
> >
> > Before this change, `PageAnchorsMetricsObserver` was recording the page
> > anchor metrics into UKM only based on `OnComplete` and
> > `FlushMetricsOnAppEnterBackground` events, and it was causing problems
> > if the user was navigating using back button. This CL fixes this issue.
> >
> > In this CL the base class for `PageAnchorsMetricsObserver::UserInteractionsData` and `PageAnchorsMetricsObserver::AnchorsData` are also changed from `WebContentsUserData` to ` DocumentUserData`.
> >
> > Bug: 1423508
> > Change-Id: Id89b845d9b8fc68fdf68f7fe0c13826ea1ec88f3
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4394306
> > Reviewed-by: Ken Okada <kenoss@chromium.org>
> > Commit-Queue: Iman Saboori <isaboori@google.com>
> > Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
> > Reviewed-by: Simon Pelchat <spelchat@chromium.org>
> > Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1135447}
>
> Bug: 1423508
> Change-Id: Ic44b130e491b0669171ae1eaae4966dca6bb523c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4484258
> Reviewed-by: Ken Okada <kenoss@chromium.org>
> Reviewed-by: Simon Pelchat <spelchat@chromium.org>
> Commit-Queue: Iman Saboori <isaboori@google.com>
> Cr-Commit-Position: refs/heads/main@{#1137809}

Bug: 1423508
Change-Id: I4e0309ee7ca708e7b55edccad5a1562f8be076d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4496117
Reviewed-by: Simon Pelchat <spelchat@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Commit-Queue: Iman Saboori <isaboori@google.com>
Cr-Commit-Position: refs/heads/main@{#1138311}