[iOS] Adds EnableCloseAllTabsConfirmation config

This CL adds the EnableCloseAllTabsConfirmation feature to the
fieldtrial testing config.

Bug: 1119319
Change-Id: I197528caa2d090e2ddd6fde43b18223da49e226f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513597


Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824884}