[ios] Enables the unstacked tabstrip when VoiceOver is on.

This works around issues where overlapped tabs confuses VoiceOver.

BUG=979803

Change-Id: I45f9fafa5b14187cc83ab935a3be385886ff8cb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933729Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719166}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -35,7 +35,7 @@ const base::Feature kDisableAnimationOnLowBattery{
     "DisableAnimationOnLowBattery", base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kVoiceOverUnstackedTabstrip{
-    "VoiceOverUnstackedTabstrip", base::FEATURE_DISABLED_BY_DEFAULT};
+    "VoiceOverUnstackedTabstrip", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kForceUnstackedTabstrip{"ForceUnstackedTabstrip",
                                             base::FEATURE_DISABLED_BY_DEFAULT};