cros: Add flag to enable pin on sign-in screen.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index aa20d19c26515fc3eea870732d608bffee6ca69e..38420c0ac6032d296bb6cbf66db79186e43abfb1 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2642,6 +2642,12 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_VALUE_TYPE(features::kCopylessPaste)},
 #endif
 
+#if defined(OS_CHROMEOS)
+    {"quick-unlock-pin-signin", flag_descriptions::kQuickUnlockPinSignin,
+     flag_descriptions::kQuickUnlockPinSigninDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(features::kQuickUnlockPinSignin)},
+#endif  // OS_CHROMEOS
+
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in
     // histograms.xml and don't forget to run AboutFlagsHistogramTest unit test.