[ios] Simplify BrowserUserData sub-classes

Since BrowserUserData stores a pointer to the owning Browser,
convert sub-classes to use this pointer instead of each of
them maintaining their own copy of the pointer.

This converts //ios/chrome/browser/sync directory.

Bug: 409299519
Change-Id: I92b30fbd6f7dd73da3ba4f77477c97dcae233af4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6573953
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1465547}