IdentityAPI: update SigninGlobalError when tokens become available

AccountTracker now kicks SigninGlobalError whenever it gets a new
token. Otherwise the error state may continue to reflect an error
that is no longer active.

BUG=310935

Review URL: https://codereview.chromium.org/39203002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230967 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230967