[iOS][coordinator] Modernize Signin Coordinator and its children.

This CL modernizes the SigninCoordinator, AddAccountSigninCoordinator,
AdvancedSettingsSigninCoordinator, UserSigninCoordinator to use
self.browser to get browserstate value.

These coordinators are already using browser in initializers.

Bug: 1029346, 1054825, 1054824, 1054823, 1054822.
Change-Id: I37a4aee333d49efc4d24fed23ffb3d1686bafb7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071939Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Cr-Commit-Position: refs/heads/master@{#744635}

ios/chrome/browser/ui/authentication/signin/add_account_signin/add_account_signin_coordinator.mm

@@ -106,10 +106,12 @@ using signin_metrics::PromoAction;
               self.browser->GetBrowserState(), self);
 
   signin::IdentityManager* identityManager =
-      IdentityManagerFactory::GetForBrowserState(self.browserState);
+      IdentityManagerFactory::GetForBrowserState(
+          self.browser->GetBrowserState());
   self.mediator = [[AddAccountSigninMediator alloc]
       initWithIdentityInteractionManager:self.identityInteractionManager
-                             prefService:self.browserState->GetPrefs()
+                             prefService:self.browser->GetBrowserState()
+                                             ->GetPrefs()
                          identityManager:identityManager];
   self.mediator.delegate = self;
   [self.mediator handleSigninIntent:self.signinIntent

ios/chrome/browser/ui/authentication/signin/signin_coordinator.h

@@ -15,8 +15,6 @@
 class Browser;
 @class ChromeIdentity;
 
-@protocol ApplicationCommands;
-@protocol BrowsingDataCommands;
 @protocol SyncPresenter;
 
 // Called when the sign-in dialog is closed.