[ios] Correctly dismiss the "Repost Form" dialog

The BrowserCoordinator did not correctly dismiss the "Report Form"
dialog when dismissing all modals (but still remove the view). Due
to that, if a new tab was opened (via an external application or a
long press on the app icon), the modal was removed but the helper
considered it still presented.

Fix BrowserCoordinator to call -stopRepostFormCoordinator when the
modals are dismissed. Additionally, change RepostFormTabHelper to
dismiss the dialog if the WebState is hidden (this should not be
necessary right now, but will prevent similar problem to appear).

Fixed: 1490764
Change-Id: I53245f4a60302991fea67c6ccdcf501d2e83dd5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5205250
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Louis Romero <lpromero@google.com>
Reviewed-by: Louis Romero <lpromero@google.com>
Cr-Commit-Position: refs/heads/main@{#1248720}