Security fix for untrusted signin confirm dialog

      When the window associated with the confirm dialog is closed without user clicking 'ok got it', chrome starts sync with default settings. This could be exploited to sign a user's Chrome into an attacker's account, as reported in crbug 321940.

BUG=321940

Review URL: https://codereview.chromium.org/79553004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237115 0039d316-1c4b-4281-b951-d872f2087c98