Adding Finch and flag for Sign-in promo

ios/chrome/browser/about_flags.mm

Index: ios/chrome/browser/about_flags.mm
diff --git a/ios/chrome/browser/about_flags.mm b/ios/chrome/browser/about_flags.mm
index c456d777769bd7f7988cab9a438c6176a277ffe8..bc0a1c4853ec941a4f288a72d16227a0257bb137 100644
--- a/ios/chrome/browser/about_flags.mm
+++ b/ios/chrome/browser/about_flags.mm
@@ -26,6 +26,7 @@
 #include "components/flags_ui/flags_storage.h"
 #include "components/flags_ui/flags_ui_switches.h"
 #include "components/ntp_tiles/switches.h"
+#include "components/signin/core/common/signin_switches.h"
 #include "components/strings/grit/components_strings.h"
 #include "ios/chrome/browser/chrome_switches.h"
 #include "ios/chrome/browser/ios_chrome_flag_descriptions.h"
@@ -231,6 +232,14 @@ void AppendSwitchesFromExperimentalSettings(base::CommandLine* command_line) {
     command_line->AppendArguments(temp_command_line, false);
   }
 
+  // Populate command line flag for Sign-in promo.
+  NSString* enableSigninPromo = [defaults stringForKey:@"EnableSigninPromo"];
+  if ([enableSigninPromo isEqualToString:@"Enabled"]) {
+    command_line->AppendSwitch(switches::kEnableSigninPromo);
+  } else if ([enableSigninPromo isEqualToString:@"Disabled"]) {
+    command_line->AppendSwitch(switches::kDisableSigninPromo);
+  }
+
   ios::GetChromeBrowserProvider()->AppendSwitchesFromExperimentalSettings(
       defaults, command_line);
 }