[Mac] Make initial visibility of the promote button on the about page deterministic

chrome/browser/ui/webui/help/version_updater_mac.mm

Index: chrome/browser/ui/webui/help/version_updater_mac.mm
diff --git a/chrome/browser/ui/webui/help/version_updater_mac.mm b/chrome/browser/ui/webui/help/version_updater_mac.mm
index 6b25716e88aa296f0ea5fb3fcaba4b4f3126ed44..130f6768a52c665668c9cad87f02e5c373734fc7 100644
--- a/chrome/browser/ui/webui/help/version_updater_mac.mm
+++ b/chrome/browser/ui/webui/help/version_updater_mac.mm
@@ -57,8 +57,9 @@ VersionUpdater* VersionUpdater::Create() {
   return new VersionUpdaterMac;
 }
 
-VersionUpdaterMac::VersionUpdaterMac() {
-  keystone_observer_.reset([[KeystoneObserver alloc] initWithUpdater:this]);
+VersionUpdaterMac::VersionUpdaterMac()
+    : show_promote_button_(false),
+      keystone_observer_([[KeystoneObserver alloc] initWithUpdater:this]) {
 }
 
 VersionUpdaterMac::~VersionUpdaterMac() {