Added finch experiment to track new menu style effect.

ui/native_theme/native_theme.cc

Index: ui/native_theme/native_theme.cc
diff --git a/ui/native_theme/native_theme.cc b/ui/native_theme/native_theme.cc
index 21071189ed6be06fb8dbdf2417ed90609b44ad88..b9a27728dd26f2c15b981e892946570c84b03296 100644
--- a/ui/native_theme/native_theme.cc
+++ b/ui/native_theme/native_theme.cc
@@ -5,8 +5,13 @@
 #include "ui/native_theme/native_theme.h"
 
 #include "base/command_line.h"
+#include "base/metrics/field_trial.h"
 #include "ui/base/ui_base_switches.h"
 
+// Constants for the new menu style field trial.
+const char kNewMenuStyleFieldTrialName[] = "NewMenuStyle";
+const char kNewMenuStyleFieldTrialGroupName[] = "OldStyle";
+
 namespace ui {
 
 void NativeTheme::SetScrollbarColors(unsigned inactive_color,
@@ -25,6 +30,12 @@ bool NativeTheme::IsNewMenuStyleEnabled() {
   static bool enable_new_menu_style =
       !CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kDisableNewMenuStyle);
+  // Run experiment only if there is no kDisableNewMenuStyle flag.
+  if (enable_new_menu_style) {
+    enable_new_menu_style =
+        base::FieldTrialList::FindFullName(kNewMenuStyleFieldTrialName) !=
+        kNewMenuStyleFieldTrialGroupName;
+  }
   return enable_new_menu_style;
 }