Allow WebStateObserver to observe N WebStates [15/N].

Convert FormInputAccessoryViewTabHelper to directly track
registration with the observed WebState instead of relying
on the deprecated code in WebStateObserver.

Bug: 775684
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I74892db41e08aae6c9025d4ee7bcdd21246f3ffa
Reviewed-on: https://chromium-review.googlesource.com/757336
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516276}

ios/chrome/browser/autofill/form_input_accessory_view_tab_helper.mm

@@ -35,11 +35,10 @@ void FormInputAccessoryViewTabHelper::CloseKeyboard() {
 FormInputAccessoryViewTabHelper::FormInputAccessoryViewTabHelper(
     web::WebState* web_state,
     NSArray<id<FormInputAccessoryViewProvider>>* providers)
-    : web::WebStateObserver(web_state),
-      controller_([[FormInputAccessoryViewController alloc]
+    : controller_([[FormInputAccessoryViewController alloc]
           initWithWebState:web_state
                  providers:providers]) {
-  DCHECK(web::WebStateObserver::web_state());
+  web_state->AddObserver(this);
 }
 
 void FormInputAccessoryViewTabHelper::WasShown(web::WebState* web_state) {
@@ -53,5 +52,6 @@ void FormInputAccessoryViewTabHelper::WasHidden(web::WebState* web_state) {
 void FormInputAccessoryViewTabHelper::WebStateDestroyed(
     web::WebState* web_state) {
   [controller_ detachFromWebState];
+  web_state->RemoveObserver(this);
   controller_ = nil;
 }