Fix race condition in Account Manager UI

A race condition exists between Account Manager's JS components'
initialization and AccountTrackerService callbacks.

Bug: 833797
Change-Id: I8eaf0d134aef4c08e089494b3ddd7d1bf7d0b2ae
Reviewed-on: https://chromium-review.googlesource.com/1150171Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577907}

chrome/browser/ui/webui/settings/chromeos/account_manager_handler.cc

@@ -157,6 +157,10 @@ void AccountManagerUIHandler::OnAccountRemoved(const AccountInfo& account_key) {
 }
 
 void AccountManagerUIHandler::RefreshUI() {
+  if (!IsJavascriptAllowed()) {
+    return;
+  }
+
   FireWebUIListener("accounts-changed");
 }