[ios] Flip the tab switcher presentation flag on by default.

Bug: 787821
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I3187e7175426f8453469283b58414618b3d4d3fb
Reviewed-on: https://chromium-review.googlesource.com/758117
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525357}

ios/chrome/browser/ui/main/main_feature_flags.cc

@@ -5,7 +5,7 @@
 #include "ios/chrome/browser/ui/main/main_feature_flags.h"
 
 const base::Feature kTabSwitcherPresentsBVC{"TabSwitcherPresentsBVC",
-                                            base::FEATURE_DISABLED_BY_DEFAULT};
+                                            base::FEATURE_ENABLED_BY_DEFAULT};
 
 bool TabSwitcherPresentsBVCEnabled() {
   return base::FeatureList::IsEnabled(kTabSwitcherPresentsBVC);