Allow WebStateObserver to observe N WebStates [14/N].

Convert AutofillTabHelper 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: If3881f8241ec52fd54499c178eb77580e2404fd6
Reviewed-on: https://chromium-review.googlesource.com/757316
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516260}

ios/chrome/browser/autofill/autofill_tab_helper.mm

@@ -36,16 +36,16 @@ id<FormSuggestionProvider> AutofillTabHelper::GetSuggestionProvider() {
 AutofillTabHelper::AutofillTabHelper(
     web::WebState* web_state,
     password_manager::PasswordGenerationManager* password_generation_manager)
-    : web::WebStateObserver(web_state),
-      controller_([[AutofillController alloc]
+    : controller_([[AutofillController alloc]
                initWithBrowserState:ios::ChromeBrowserState::FromBrowserState(
                                         web_state->GetBrowserState())
           passwordGenerationManager:password_generation_manager
                            webState:web_state]) {
-  DCHECK(web::WebStateObserver::web_state());
+  web_state->AddObserver(this);
 }
 
 void AutofillTabHelper::WebStateDestroyed(web::WebState* web_state) {
   [controller_ detachFromWebState];
+  web_state->RemoveObserver(this);
   controller_ = nil;
 }