[DIPS] Move IncrementPageSpecificBounceCount() out of DIPSWebContentsObserver.

DIPSWebContentsObserver::IncrementPageSpecificBounceCount() calls
PageSpecificContentSettings::IncrementStatefulBounceCount(), but
PageSpecificContentSettings is not in //content. So when DIPSService
moves to //content, it will not be able to call it anymore.

TL;DR: Move that call to a new class that observes
DIPSWebContentsObserver.

* Declare DIPSWebContentsObserver::Observer class with OnStatefulBounce() method
* Add DIPSWebContentsObserver::AddObserver/RemoveObserver() methods
* Replace DIPSWebContentsObserver::IncrementPageSpecificBounceCount() with OnStatefulBounce() that just notifies observers
* Rename various parameters from content_settings_callback to stateful_bounce_callback
* Make DIPSWebContentsObserver subclass base::SupportsUserData (to manage standalone observers' lifetimes)
* Add DIPSWebContentsObserver::Observer subclass StatefulBounceCounter for calling PageSpecificContentSettings::IncrementStatefulBounceCount()
* Make TabHelpers::AttachTabHelpers() create StatefulBounceCounter

Bug: 343631048
Change-Id: Ia7c1bb02797c04efc960383e36ce28a150837a91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5849530
Commit-Queue: Ryan Tarpine <rtarpine@chromium.org>
Reviewed-by: Joshua Hood <jdh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1354130}