Perform /ListAccounts check before session merge to see if there is a need for session merge at all.

BUG=338543
TEST=manual, added OAuth2Test.PRE_PRE_MergeSession to test existing user with fresh cookies, modified OAuth2Test.PRE_MergeSession to test existing user with stale GAIA cookies

Review URL: https://codereview.chromium.org/148463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247862 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247862