[iOS] Implement add account button for UnifiedConsentCoordinator.

Implements the add account button tap event in the
UnifiedConsentCoordinator, which opens the add account flow.

See go/chrome-ios-signin-migration for more information.

Bug: 971989
Change-Id: I51b167da48deb8092bbab139ed2fce34a4cac194
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062410
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742713}

ios/chrome/browser/ui/authentication/signin/add_account_signin/BUILD.gn

@@ -21,7 +21,6 @@ source_set("add_account_signin") {
     "//ios/chrome/browser/ui/alert_coordinator",
     "//ios/chrome/browser/ui/authentication",
     "//ios/chrome/browser/ui/authentication/signin:signin_protected",
-    "//ios/chrome/browser/ui/authentication/signin/user_signin",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",
   ]

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

@@ -13,7 +13,6 @@
 #import "ios/chrome/browser/ui/authentication/authentication_ui_util.h"
 #import "ios/chrome/browser/ui/authentication/signin/add_account_signin/add_account_signin_mediator.h"
 #import "ios/chrome/browser/ui/authentication/signin/signin_coordinator+protected.h"
-#import "ios/chrome/browser/ui/authentication/signin/user_signin/user_signin_coordinator.h"
 #import "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 #import "ios/public/provider/chrome/browser/signin/chrome_identity_interaction_manager.h"
 #import "ios/public/provider/chrome/browser/signin/chrome_identity_service.h"

ios/chrome/browser/ui/authentication/signin/user_signin/user_signin_coordinator.mm

@@ -20,6 +20,8 @@ using signin_metrics::PromoAction;
 // Coordinator that handles the user consent before the user signs in.
 @property(nonatomic, strong)
     UnifiedConsentCoordinator* unifiedConsentCoordinator;
+// Coordinator that handles adding a user account.
+@property(nonatomic, strong) SigninCoordinator* addAccountSigninCoordinator;
 // View controller that handles the sign-in UI.
 @property(nonatomic, strong) UserSigninViewController* viewController;
 // Suggested identity shown at sign-in.
@@ -65,6 +67,19 @@ using signin_metrics::PromoAction;
 
   [self.unifiedConsentCoordinator start];
 
+  self.addAccountSigninCoordinator = [SigninCoordinator
+      addAccountCoordinatorWithBaseViewController:self.viewController
+                                          browser:self.browser
+                                      accessPoint:self.accessPoint];
+
+  __weak UserSigninCoordinator* weakSelf = self;
+  self.addAccountSigninCoordinator.signinCompletion =
+      ^(SigninCoordinatorResult signinResult, ChromeIdentity* identity) {
+        if (signinResult == SigninCoordinatorResultSuccess) {
+          weakSelf.defaultIdentity = identity;
+        }
+      };
+
   // Display UnifiedConsentViewController within the host.
   self.viewController.unifiedConsentViewController =
       self.unifiedConsentCoordinator.viewController;
@@ -75,6 +90,8 @@ using signin_metrics::PromoAction;
 
 - (void)stop {
   [super stop];
+  [self.addAccountSigninCoordinator stop];
+  self.addAccountSigninCoordinator = nil;
   self.unifiedConsentCoordinator = nil;
 }
 
@@ -92,7 +109,8 @@ using signin_metrics::PromoAction;
 
 - (void)unifiedConsentCoordinatorDidTapOnAddAccount:
     (UnifiedConsentCoordinator*)coordinator {
-  // TODO(crbug.com/971989): Needs implementation.
+  DCHECK_EQ(self.unifiedConsentCoordinator, coordinator);
+  [self.addAccountSigninCoordinator start];
 }
 
 - (void)unifiedConsentCoordinatorNeedPrimaryButtonUpdate: