[ios] Turn on Default Browser Fullscreen promo feature on by default

Bug: 1107489
Change-Id: Ib5686a3200dbd2783854f8dc19dbd9c3618d8f8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512382
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827766}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -61,7 +61,7 @@ const base::Feature kEnableFullPageScreenshot{
     "EnableFullPageScreenshot", base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kDefaultBrowserFullscreenPromo{
-    "DefaultBrowserFullscreenPromo", base::FEATURE_DISABLED_BY_DEFAULT};
+    "DefaultBrowserFullscreenPromo", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kDefaultBrowserSettings{"DefaultBrowserSettings",
                                             base::FEATURE_ENABLED_BY_DEFAULT};