[IOS] Adds eg2test related to EnableCloseAllTabsConfirmation feature

Tests that tapping on "Close All" shows a confirmation dialog.
It also tests that tapping on "Close x Tab(s)" on the confirmation
dialog displays an empty grid and tapping on "Cancel" doesn't modify
the grid.

Bug: 1119319, 1124309
Change-Id: I65a8a70a867efe3643ebfde1c59f3fb186177c32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450317


Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824895}