[iOS] Add missing observer at init

An observer was missing on this class. Found while hitting a DCHECK.
Most likely the product of a merge conflict.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I29fb74d9ac2a0d6404748e592089f80f4ec6f9b5
Reviewed-on: https://chromium-review.googlesource.com/1174264Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582915}

ios/chrome/browser/ui/autofill/form_input_accessory_mediator.mm

@@ -98,6 +98,9 @@
         _formActivityObserverBridge =
             std::make_unique<autofill::FormActivityObserverBridge>(_webState,
                                                                    self);
+        _webStateObserverBridge =
+            std::make_unique<web::WebStateObserverBridge>(self);
+        webState->AddObserver(_webStateObserverBridge.get());
       }
     }
     _formInputAccessoryHandler = [[FormInputAccessoryViewHandler alloc] init];