[iOS] Observe WebViewProxyTabHelper in FullscreenWebStateObserver

If `kSmoothScrollingDefault` is disabled then FullscreenWebStateObserver
is responsible for updating the FullscreenModel through its
FullscreenWebViewProxyObserver.

This CL makes FullscreenWebStateObserver observer the WebState's
WebViewProxyTabHelper so it can feed the correct proxy to the proxy
observer.

Bug: 417688581
Change-Id: Ida497debe8e174f73a83fe3b5922f5aa515171fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6718039
Auto-Submit: Quentin Pubert <qpubert@google.com>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1485593}