[iOS] Remove scroll view size broadcasting

Move scroll view size broadcasting in smooth scrolling experiment only
and replace it by a call to set the scroll view size only after the page
loaded.

Pair-programming: ajuma@

Change-Id: I8bf11b27b05104b28fd69a19d9eeb8d7bc77b896
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4834439
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Aliona Dangla <alionadangla@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1196082}