Allow WebStateObserver to observe N WebStates [35/N].

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

Bug: 775684
Change-Id: Ifdf8f2ba5e113f67269d4329500d1693e772eeae
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/768877
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517793}

ios/web_view/internal/translate/web_view_translate_client.mm

@@ -46,8 +46,7 @@ WebViewBrowserState* GetMainBrowserState() {
 }  // namespace
 
 WebViewTranslateClient::WebViewTranslateClient(web::WebState* web_state)
-    : web::WebStateObserver(web_state),
-      translate_manager_(base::MakeUnique<translate::TranslateManager>(
+    : translate_manager_(base::MakeUnique<translate::TranslateManager>(
           this,
           WebViewTranslateRankerFactory::GetForBrowserState(
               WebViewBrowserState::FromBrowserState(GetMainBrowserState())),
@@ -55,7 +54,9 @@ WebViewTranslateClient::WebViewTranslateClient(web::WebState* web_state)
               WebViewBrowserState::FromBrowserState(GetMainBrowserState())))),
       translate_driver_(web_state,
                         web_state->GetNavigationManager(),
-                        translate_manager_.get()) {}
+                        translate_manager_.get()) {
+  web_state->AddObserver(this);
+}
 
 WebViewTranslateClient::~WebViewTranslateClient() = default;
 
@@ -90,7 +91,6 @@ PrefService* WebViewTranslateClient::GetPrefs() {
 
 std::unique_ptr<translate::TranslatePrefs>
 WebViewTranslateClient::GetTranslatePrefs() {
-  DCHECK(web_state());
   return base::MakeUnique<translate::TranslatePrefs>(
       GetPrefs(), prefs::kAcceptLanguages, nullptr);
 }
@@ -129,6 +129,7 @@ void WebViewTranslateClient::ShowReportLanguageDetectionErrorUI(
 }
 
 void WebViewTranslateClient::WebStateDestroyed(web::WebState* web_state) {
+  web_state->RemoveObserver(this);
   // Translation process can be interrupted.
   // Destroying the TranslateManager now guarantees that it never has to deal
   // with nullptr WebState.