[iOS] Switch to using the new sign-in architecture by default.

Enables the new sign-in architecture using the Coordinator-Mediator
design paradigm for Chrome iOS. For more details see
go/chrome-ios-signin-migration.

Bug: 971989
Change-Id: I0710436f74e14b7a241e25e3b234722a9add37fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157128
Commit-Queue: Nohemi Fernandez <[email protected]>
Reviewed-by: Jérôme Lebel <[email protected]>
Cr-Commit-Position: refs/heads/master@{#764354}
diff --git a/ios/chrome/browser/ui/ui_feature_flags.cc b/ios/chrome/browser/ui/ui_feature_flags.cc
index 763a6d1..b40c05c 100644
--- a/ios/chrome/browser/ui/ui_feature_flags.cc
+++ b/ios/chrome/browser/ui/ui_feature_flags.cc
@@ -38,7 +38,7 @@
     "FirstResponderSendAction", base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kNewSigninArchitecture{"NewSigninArchitecture",
-                                           base::FEATURE_DISABLED_BY_DEFAULT};
+                                           base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kQRCodeGeneration{"QRCodeGeneration",
                                       base::FEATURE_DISABLED_BY_DEFAULT};