[ios] Clear Browser's UserData at the end of the destructor

Most of Browser's UserData objects have a pointer back to the
Browser. As base::SupportUserData is a super class of Browser
the base::SupportUserData::UserData were being destroyed after
the vtable of Browser cleared. This made calling methods on
Browser really risky in Browser's UserData destructors.

To make this safer, call ClearAllUserData() at the end of the
destructor of BrowserImpl, ensuring that all the UserData are
destroyed before the object itself becomes invalid. This means
that the UserData object can keep a simple raw_ptr<Browser>
and be sure that it will be valid for their whole lifetime.

Bug: 409299519
Change-Id: I9c4b59180dd7993df9b969d4a5d4b8069c0351c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6516292
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456260}