[iOS] Enable BVC being contained

This CL enables the feature flag to have the BVC being contained
instead of presented.

Fixed: 1038034
Change-Id: Id764ee5b8fd26c8c71ae694ea69b8394a8507c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108734Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751961}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -26,7 +26,7 @@ const base::Feature kForceUnstackedTabstrip{"ForceUnstackedTabstrip",
                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kContainedBVC{"ContainedBVC",
-                                  base::FEATURE_DISABLED_BY_DEFAULT};
+                                  base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kTestFeature{"TestFeature",
                                  base::FEATURE_DISABLED_BY_DEFAULT};