[iOS] Have the BVC being contained by default

This enables the flag by default on Canary.
The idea is to ship it to 100%, with a kill switch by Finch.

Bug: 1038034
Change-Id: I0ea6097bd844db1940eff683d99d555f1654189d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062979
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744811}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -29,7 +29,7 @@ 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};