Add convenience methods for checking horizontal size class.

ios/chrome/browser/ui/uikit_ui_util.mm

Index: ios/chrome/browser/ui/uikit_ui_util.mm
diff --git a/ios/chrome/browser/ui/uikit_ui_util.mm b/ios/chrome/browser/ui/uikit_ui_util.mm
index 6f0ac49dac794b4a47e4a36c43629eda3306ec93..f5b315c898617af9d368cf58e42cda11083b4328 100644
--- a/ios/chrome/browser/ui/uikit_ui_util.mm
+++ b/ios/chrome/browser/ui/uikit_ui_util.mm
@@ -517,10 +517,9 @@ void AddSameCenterYConstraint(UIView* parentView,
                                           constant:0]];
 }
 
-bool IsCompact() {
+bool IsCompact(id<UITraitEnvironment> environment) {
   if (base::ios::IsRunningOnIOS8OrLater()) {
-    UIWindow* keyWindow = [UIApplication sharedApplication].keyWindow;
-    return [keyWindow.traitCollection horizontalSizeClass] ==
+    return environment.traitCollection.horizontalSizeClass ==
            UIUserInterfaceSizeClassCompact;
   } else {
     // Prior to iOS 8, iPad is always regular, iPhone is always compact.
@@ -528,6 +527,15 @@ bool IsCompact() {
   }
 }
 
+bool IsCompact() {
+  UIWindow* keyWindow = [UIApplication sharedApplication].keyWindow;
+  return IsCompact(keyWindow);
+}
+
+bool IsCompactTablet(id<UITraitEnvironment> environment) {
+  return IsIPadIdiom() && IsCompact(environment);
+}
+
 bool IsCompactTablet() {
   return IsIPadIdiom() && IsCompact();
 }