Add a policy to always show the Cast toolbar icon

chrome/browser/policy/configuration_policy_handler_list_factory.cc

Index: chrome/browser/policy/configuration_policy_handler_list_factory.cc
diff --git a/chrome/browser/policy/configuration_policy_handler_list_factory.cc b/chrome/browser/policy/configuration_policy_handler_list_factory.cc
index d419d89617ab967cc393b067617d191c8a235dd0..64b19a63b2917ddbf7fed5252827f01e70f6880f 100644
--- a/chrome/browser/policy/configuration_policy_handler_list_factory.cc
+++ b/chrome/browser/policy/configuration_policy_handler_list_factory.cc
@@ -410,6 +410,11 @@ const PolicyToPreferenceMapEntry kSimplePolicyMap[] = {
   { key::kEnableMediaRouter,
     prefs::kEnableMediaRouter,
     base::Value::Type::BOOLEAN },
+#if !defined(OS_ANDROID)
+  { key::kShowCastIconInToolbar,
+    prefs::kShowCastIconInToolbar,
+    base::Value::Type::BOOLEAN },
+#endif  // !defined(OS_ANDROID)
 #endif  // defined(ENABLE_MEDIA_ROUTER)
 #if BUILDFLAG(ENABLE_WEBRTC)
   { key::kWebRtcUdpPortRange,