OneClickSigninHelper::CanOffer checks IsEnableWebBasedSignin

Before, the CanOffer method could return true, while the web based signin was disabled through about:flags. This meant that neither the save password prompt, nor the sign-in interstitial were shown.

This CL ensures that CanOffer returns false if the web based signing is disabled in about:flags.

BUG=339804

Review URL: https://codereview.chromium.org/160363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251125 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251125