[ios] Disables Contained BVC by default.

gambard@ decided to disable for now since there were some issues like
https://crrev.com/c/2090804

TBR = gambard@chromium.org

Bug: 1038034
Change-Id: Id6e6c0be0f907efe9765c873f48f1a21583c34d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091898Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747856}

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_ENABLED_BY_DEFAULT};
+                                  base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kTestFeature{"TestFeature",
                                  base::FEATURE_DISABLED_BY_DEFAULT};