[ios] Add IsInFirstRunModalGroup() check for location prompt trigger

For both IsInRemoveFirstRunPromptGroup() and IsInFirstRunModalGroup()
experiment groups the native location prompt should not show after First
Run finishes. The prompt will show after the user taps the action button
in the modal.

Bug: 1146456
Change-Id: I65dad9dcd3e46e45d783df79e1f3fe9aa5d2ea52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537922
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827422}

ios/chrome/browser/ui/main/scene_controller.mm

@@ -909,7 +909,8 @@ const char kMultiWindowOpenInNewWindowHistogram[] =
                 name:kChromeFirstRunUIDidFinishNotification
               object:nil];
 
-  if (!location_permissions_field_trial::IsInRemoveFirstRunPromptGroup()) {
+  if (!location_permissions_field_trial::IsInRemoveFirstRunPromptGroup() &&
+      !location_permissions_field_trial::IsInFirstRunModalGroup()) {
     // As soon as First Run has finished, give OmniboxGeolocationController an
     // opportunity to present the iOS system location alert.
     [[OmniboxGeolocationController sharedInstance]