[ios] Fix default browser fullscreen promo experiment retrieval

GetFieldTrial() is the correct API to call for finch config experiments.

Bug: 1165875
Change-Id: Ic3a0657c1ee2c6e1bb815c91607ec12b0ed41afa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626104Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843187}

ios/chrome/browser/ui/whats_new/default_browser_utils.mm

@@ -97,16 +97,18 @@ bool ShouldShowRemindMeLaterDefaultBrowserFullscreenPromo() {
 bool IsInRemindMeLaterGroup() {
   return base::FeatureList::IsEnabled(
              kDefaultBrowserFullscreenPromoExperiment) &&
-         base::FieldTrialList::FindFullName(
-             kDefaultBrowserFullscreenPromoExperiment.name) ==
+         base::FeatureList::GetFieldTrial(
+             kDefaultBrowserFullscreenPromoExperiment)
+                 ->group_name() ==
              kDefaultBrowserFullscreenPromoExperimentRemindMeGroupName;
 }
 
 bool IsInModifiedStringsGroup() {
   return base::FeatureList::IsEnabled(
              kDefaultBrowserFullscreenPromoExperiment) &&
-         base::FieldTrialList::FindFullName(
-             kDefaultBrowserFullscreenPromoExperiment.name) ==
+         base::FeatureList::GetFieldTrial(
+             kDefaultBrowserFullscreenPromoExperiment)
+                 ->group_name() ==
              kDefaultBrowserFullscreenPromoExperimentChangeStringsGroupName;
 }