Allow WebStateObserver to observe N WebStates [26/N].

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

Bug: 775684
Change-Id: I5add60c72e6046400c2dc1c4a609e9f443ecb782
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/768420
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517049}

ios/chrome/browser/ui/dialogs/java_script_dialog_blocking_state.h

@@ -47,6 +47,10 @@ class JavaScriptDialogBlockingState
                           web::NavigationContext* navigation_context) override;
   void WebStateDestroyed(web::WebState* web_state) override;
 
+  // The WebState this instance is observing. Will be null after
+  // WebStateDestroyed has been called.
+  web::WebState* web_state_ = nullptr;
+
   // Whether to show the blocking option.
   size_t dialog_count_ = 0;
   // The NavigationItem for which JavaScript dialogs were blocked.

ios/chrome/browser/ui/dialogs/java_script_dialog_blocking_state.mm

@@ -4,6 +4,7 @@
 
 #import "ios/chrome/browser/ui/dialogs/java_script_dialog_blocking_state.h"
 
+#include "base/logging.h"
 #import "ios/web/public/navigation_item.h"
 #import "ios/web/public/navigation_manager.h"
 #import "ios/web/public/web_state/navigation_context.h"
@@ -16,24 +17,25 @@ DEFINE_WEB_STATE_USER_DATA_KEY(JavaScriptDialogBlockingState);
 
 JavaScriptDialogBlockingState::JavaScriptDialogBlockingState(
     web::WebState* web_state)
-    : web::WebStateObserver(web_state) {
-  DCHECK(web_state);
+    : web_state_(web_state) {
+  web_state_->AddObserver(this);
 }
 
 JavaScriptDialogBlockingState::~JavaScriptDialogBlockingState() {
   // It is expected that WebStateDestroyed() will be received before this state
   // is deallocated.
-  DCHECK(!web_state());
+  DCHECK(!web_state_);
 }
 
 void JavaScriptDialogBlockingState::JavaScriptDialogBlockingOptionSelected() {
-  blocked_item_ = web_state()->GetNavigationManager()->GetLastCommittedItem();
+  blocked_item_ = web_state_->GetNavigationManager()->GetLastCommittedItem();
   DCHECK(blocked_item_);
 }
 
 void JavaScriptDialogBlockingState::DidStartNavigation(
     web::WebState* web_state,
     web::NavigationContext* navigation_context) {
+  DCHECK_EQ(web_state_, web_state);
   web::NavigationItem* item =
       web_state->GetNavigationManager()->GetLastCommittedItem();
   // The dialog blocking state should be reset for user-initiated loads or for
@@ -47,5 +49,7 @@ void JavaScriptDialogBlockingState::DidStartNavigation(
 
 void JavaScriptDialogBlockingState::WebStateDestroyed(
     web::WebState* web_state) {
-  Observe(nullptr);
+  DCHECK_EQ(web_state_, web_state);
+  web_state_->RemoveObserver(this);
+  web_state_ = nullptr;
 }